2025-10-31
3289. The Two Sneaky Numbers of Digitville

Topic: Array, Hash Table, Math
Difficulty: Easy

Problem:
In the town of Digitville, there was a list of numbers called nums containing integers from 0 to n - 1. Each number was supposed to appear exactly once in the list, however, two mischievous numbers sneaked in an additional time, making the list longer than usual.

As the town detective, your task is to find these two sneaky numbers. Return an array of size two containing the two numbers (in any order), so peace can return to Digitville.

Example 1:

Input: nums = 0,1,1,0

Output: 0,1

Explanation:

The numbers 0 and 1 each appear twice in the array.

Example 2:

Input: nums = 0,3,2,1,3,2

Output: 2,3

Explanation:

The numbers 2 and 3 each appear twice in the array.

Example 3:

Input: nums = 7,1,5,4,3,4,6,0,9,5,8,2

Output: 4,5

Explanation:

The numbers 4 and 5 each appear twice in the array.

Constraints:

• 2 <= n <= 100
• nums.length == n + 2
• 0 <= nums[i] < n
• The input is generated such that nums contains exactly two repeated elements.

2025-11-01
3217. Delete Nodes From Linked List Present in Array

Topic: Array, Hash Table, Linked List
Difficulty: Medium

Problem:
You are given an array of integers nums and the head of a linked list. Return the head of the modified linked list after removing all nodes from the linked list that have a value that exists in nums.

Example 1:

Input: nums = 1,2,3, head = 1,2,3,4,5

Output: 4,5

Explanation:

Image: https://assets.leetcode.com/uploads/2024/06/11/linkedlistexample0.png

Remove the nodes with values 1, 2, and 3.

Example 2:

Input: nums = 1, head = 1,2,1,2,1,2

Output: 2,2,2

Explanation:

Image: https://assets.leetcode.com/uploads/2024/06/11/linkedlistexample1.png

Remove the nodes with value 1.

Example 3:

Input: nums = 5, head = 1,2,3,4

Output: 1,2,3,4

Explanation:

Image: https://assets.leetcode.com/uploads/2024/06/11/linkedlistexample2.png

No node has value 5.

Constraints:

• 1 <= nums.length <= 10^5
• 1 <= nums[i] <= 10^5
• All elements in nums are unique.
• The number of nodes in the given list is in the range [1, 10^5].
• 1 <= Node.val <= 10^5
• The input is generated such that there is at least one node in the linked list that has a value not present in nums.

2025-11-02
2257. Count Unguarded Cells in the Grid

Topic: Array, Matrix, Simulation
Difficulty: Medium

Problem:
You are given two integers m and n representing a 0-indexed m x n grid. You are also given two 2D integer arrays guards and walls where guards[i] = [row_i, col_i] and walls[j] = [row_j, col_j] represent the positions of the i^th guard and j^th wall respectively.

A guard can see every cell in the four cardinal directions (north, east, south, or west) starting from their position unless obstructed by a wall or another guard. A cell is guarded if there is at least one guard that can see it.

Return the number of unoccupied cells that are not guarded.

Example 1:

Image: https://assets.leetcode.com/uploads/2022/03/10/example1drawio2.png

Input: m = 4, n = 6, guards = [[0,0],[1,1],[2,3]], walls = [[0,1],[2,2],[1,4]]
Output: 7
Explanation: The guarded and unguarded cells are shown in red and green respectively in the above diagram.
There are a total of 7 unguarded cells, so we return 7.

Example 2:

Image: https://assets.leetcode.com/uploads/2022/03/10/example2drawio.png

Input: m = 3, n = 3, guards = [[1,1]], walls = [[0,1],[1,0],[2,1],[1,2]]
Output: 4
Explanation: The unguarded cells are shown in green in the above diagram.
There are a total of 4 unguarded cells, so we return 4.

Constraints:

• 1 <= m, n <= 10^5
• 2 <= m * n <= 10^5
• 1 <= guards.length, walls.length <= 5 * 10^4
• 2 <= guards.length + walls.length <= m * n
• guards[i].length == walls[j].length == 2
• 0 <= row_i, row_j < m
• 0 <= col_i, col_j < n
• All the positions in guards and walls are unique.

2025-11-03
1578. Minimum Time to Make Rope Colorful

Topic: Array, String, Dynamic Programming, Greedy
Difficulty: Medium

Problem:
Alice has n balloons arranged on a rope. You are given a 0-indexed string colors where colors[i] is the color of the i^th balloon.

Alice wants the rope to be colorful. She does not want two consecutive balloons to be of the same color, so she asks Bob for help. Bob can remove some balloons from the rope to make it colorful. You are given a 0-indexed integer array neededTime where neededTime[i] is the time (in seconds) that Bob needs to remove the i^th balloon from the rope.

Return the minimum time Bob needs to make the rope colorful.

Example 1:

Image: https://assets.leetcode.com/uploads/2021/12/13/ballon1.jpg

Input: colors = "abaac", neededTime = [1,2,3,4,5]
Output: 3
Explanation: In the above image, 'a' is blue, 'b' is red, and 'c' is green.
Bob can remove the blue balloon at index 2. This takes 3 seconds.
There are no longer two consecutive balloons of the same color. Total time = 3.

Example 2:

Image: https://assets.leetcode.com/uploads/2021/12/13/balloon2.jpg

Input: colors = "abc", neededTime = [1,2,3]
Output: 0
Explanation: The rope is already colorful. Bob does not need to remove any balloons from the rope.

Example 3:

Image: https://assets.leetcode.com/uploads/2021/12/13/balloon3.jpg

Input: colors = "aabaa", neededTime = [1,2,3,4,1]
Output: 2
Explanation: Bob will remove the balloons at indices 0 and 4. Each balloons takes 1 second to remove.
There are no longer two consecutive balloons of the same color. Total time = 1 + 1 = 2.

Constraints:

• n == colors.length == neededTime.length
• 1 <= n <= 10^5
• 1 <= neededTime[i] <= 10^4
• colors contains only lowercase English letters.

2025-11-04
3318. Find X-Sum of All K-Long Subarrays I

Topic: Array, Hash Table, Sliding Window, Heap (Priority Queue)
Difficulty: Easy

Problem:
You are given an array nums of n integers and two integers k and x.

The x-sum of an array is calculated by the following procedure:

• Count the occurrences of all elements in the array.
• Keep only the occurrences of the top x most frequent elements. If two elements have the same number of occurrences, the element with the bigger value is considered more frequent.
• Calculate the sum of the resulting array.

Note that if an array has less than x distinct elements, its x-sum is the sum of the array.

Return an integer array answer of length n - k + 1 where answer[i] is the x-sum of the subarray nums[i..i + k - 1].

Example 1:

Input: nums = 1,1,2,2,3,4,2,3, k = 6, x = 2

Output: 6,10,12

Explanation:

• For subarray [1, 1, 2, 2, 3, 4], only elements 1 and 2 will be kept in the resulting array. Hence, answer[0] = 1 + 1 + 2 + 2.
• For subarray [1, 2, 2, 3, 4, 2], only elements 2 and 4 will be kept in the resulting array. Hence, answer[1] = 2 + 2 + 2 + 4. Note that 4 is kept in the array since it is bigger than 3 and 1 which occur the same number of times.
• For subarray [2, 2, 3, 4, 2, 3], only elements 2 and 3 are kept in the resulting array. Hence, answer[2] = 2 + 2 + 2 + 3 + 3.

Example 2:

Input: nums = 3,8,7,8,7,5, k = 2, x = 2

Output: 11,15,15,15,12

Explanation:

Since k == x, answer[i] is equal to the sum of the subarray nums[i..i + k - 1].

Constraints:

• 1 <= n == nums.length <= 50
• 1 <= nums[i] <= 50
• 1 <= x <= k <= nums.length

2025-11-05
3321. Find X-Sum of All K-Long Subarrays II

Topic: Array, Hash Table, Sliding Window, Heap (Priority Queue)
Difficulty: Hard

Problem:
You are given an array nums of n integers and two integers k and x.

The x-sum of an array is calculated by the following procedure:

• Count the occurrences of all elements in the array.
• Keep only the occurrences of the top x most frequent elements. If two elements have the same number of occurrences, the element with the bigger value is considered more frequent.
• Calculate the sum of the resulting array.

Note that if an array has less than x distinct elements, its x-sum is the sum of the array.

Return an integer array answer of length n - k + 1 where answer[i] is the x-sum of the subarray nums[i..i + k - 1].

Example 1:

Input: nums = 1,1,2,2,3,4,2,3, k = 6, x = 2

Output: 6,10,12

Explanation:

• For subarray [1, 1, 2, 2, 3, 4], only elements 1 and 2 will be kept in the resulting array. Hence, answer[0] = 1 + 1 + 2 + 2.
• For subarray [1, 2, 2, 3, 4, 2], only elements 2 and 4 will be kept in the resulting array. Hence, answer[1] = 2 + 2 + 2 + 4. Note that 4 is kept in the array since it is bigger than 3 and 1 which occur the same number of times.
• For subarray [2, 2, 3, 4, 2, 3], only elements 2 and 3 are kept in the resulting array. Hence, answer[2] = 2 + 2 + 2 + 3 + 3.

Example 2:

Input: nums = 3,8,7,8,7,5, k = 2, x = 2

Output: 11,15,15,15,12

Explanation:

Since k == x, answer[i] is equal to the sum of the subarray nums[i..i + k - 1].

Constraints:

• nums.length == n
• 1 <= n <= 10^5
• 1 <= nums[i] <= 10^9
• 1 <= x <= k <= nums.length

2025-11-06
3607. Power Grid Maintenance

Topic: Array, Hash Table, Depth-First Search, Breadth-First Search, Union Find, Graph, Heap (Priority Queue), Ordered Set
Difficulty: Medium

Problem:
You are given an integer c representing c power stations, each with a unique identifier id from 1 to c (1‑based indexing).

These stations are interconnected via n bidirectional cables, represented by a 2D array connections, where each element connections[i] = [u_i, v_i] indicates a connection between station u_i and station v_i. Stations that are directly or indirectly connected form a power grid.

Initially, all stations are online (operational).

You are also given a 2D array queries, where each query is one of the following two types:

• [1, x]: A maintenance check is requested for station x. If station x is online, it resolves the check by itself. If station x is offline, the check is resolved by the operational station with the smallest id in the same power grid as x. If no operational station exists in that grid, return -1.
• [2, x]: Station x goes offline (i.e., it becomes non-operational).

Return an array of integers representing the results of each query of type [1, x] in the order they appear.

Note: The power grid preserves its structure; an offline (non‑operational) node remains part of its grid and taking it offline does not alter connectivity.

Example 1:

Input: c = 5, connections = [1,2,2,3,3,4,4,5], queries = [1,3,2,1,1,1,2,2,1,2]

Output: 3,2,3

Explanation:

Image: https://assets.leetcode.com/uploads/2025/04/15/powergrid.jpg

• Initially, all stations {1, 2, 3, 4, 5} are online and form a single power grid.
• Query [1,3]: Station 3 is online, so the maintenance check is resolved by station 3.
• Query [2,1]: Station 1 goes offline. The remaining online stations are {2, 3, 4, 5}.
• Query [1,1]: Station 1 is offline, so the check is resolved by the operational station with the smallest id among {2, 3, 4, 5}, which is station 2.
• Query [2,2]: Station 2 goes offline. The remaining online stations are {3, 4, 5}.
• Query [1,2]: Station 2 is offline, so the check is resolved by the operational station with the smallest id among {3, 4, 5}, which is station 3.

Example 2:

Input: c = 3, connections = , queries = [1,1,2,1,1,1]

Output: 1,-1

Explanation:

• There are no connections, so each station is its own isolated grid.
• Query [1,1]: Station 1 is online in its isolated grid, so the maintenance check is resolved by station 1.
• Query [2,1]: Station 1 goes offline.
• Query [1,1]: Station 1 is offline and there are no other stations in its grid, so the result is -1.

Constraints:

• 1 <= c <= 10^5
• 0 <= n == connections.length <= min(10^5, c * (c - 1) / 2)
• connections[i].length == 2
• 1 <= u_i, v_i <= c
• u_i != v_i
• 1 <= queries.length <= 2 * 10^5
• queries[i].length == 2
• queries[i][0] is either 1 or 2.
• 1 <= queries[i][1] <= c

2025-11-07
2528. Maximize the Minimum Powered City

Topic: Array, Binary Search, Greedy, Queue, Sliding Window, Prefix Sum
Difficulty: Hard

Problem:
You are given a 0-indexed integer array stations of length n, where stations[i] represents the number of power stations in the i^th city.

Each power station can provide power to every city in a fixed range. In other words, if the range is denoted by r, then a power station at city i can provide power to all cities j such that |i - j| <= r and 0 <= i, j <= n - 1.

• Note that |x| denotes absolute value. For example, |7 - 5| = 2 and |3 - 10| = 7.

The power of a city is the total number of power stations it is being provided power from.

The government has sanctioned building k more power stations, each of which can be built in any city, and have the same range as the pre-existing ones.

Given the two integers r and k, return the maximum possible minimum power of a city, if the additional power stations are built optimally.

Note that you can build the k power stations in multiple cities.

Example 1:

Input: stations = [1,2,4,5,0], r = 1, k = 2
Output: 5
Explanation: 
One of the optimal ways is to install both the power stations at city 1. 
So stations will become [1,4,4,5,0].
- City 0 is provided by 1 + 4 = 5 power stations.
- City 1 is provided by 1 + 4 + 4 = 9 power stations.
- City 2 is provided by 4 + 4 + 5 = 13 power stations.
- City 3 is provided by 5 + 4 = 9 power stations.
- City 4 is provided by 5 + 0 = 5 power stations.
So the minimum power of a city is 5.
Since it is not possible to obtain a larger power, we return 5.

Example 2:

Input: stations = [4,4,4,4], r = 0, k = 3
Output: 4
Explanation: 
It can be proved that we cannot make the minimum power of a city greater than 4.

Constraints:

• n == stations.length
• 1 <= n <= 10^5
• 0 <= stations[i] <= 10^5
• 0 <= r <= n - 1
• 0 <= k <= 10^9

2025-11-08
1611. Minimum One Bit Operations to Make Integers Zero

Topic: Dynamic Programming, Bit Manipulation, Memoization
Difficulty: Hard

Problem:
Given an integer n, you must transform it into 0 using the following operations any number of times:

• Change the rightmost (0^th) bit in the binary representation of n.
• Change the i^th bit in the binary representation of n if the (i-1)^th bit is set to 1 and the (i-2)^th through 0^th bits are set to 0.

Return the minimum number of operations to transform n into 0.

Example 1:

Input: n = 3
Output: 2
Explanation: The binary representation of 3 is "11".
"11" -> "01" with the 2^nd operation since the 0^th bit is 1.
"01" -> "00" with the 1^st operation.

Example 2:

Input: n = 6
Output: 4
Explanation: The binary representation of 6 is "110".
"110" -> "010" with the 2^nd operation since the 1^st bit is 1 and 0^th through 0^th bits are 0.
"010" -> "011" with the 1^st operation.
"011" -> "001" with the 2^nd operation since the 0^th bit is 1.
"001" -> "000" with the 1^st operation.

Constraints:

• 0 <= n <= 10^9

2025-11-09
2169. Count Operations to Obtain Zero

Topic: Math, Simulation
Difficulty: Easy

Problem:
You are given two non-negative integers num1 and num2.

In one operation, if num1 >= num2, you must subtract num2 from num1, otherwise subtract num1 from num2.

• For example, if num1 = 5 and num2 = 4, subtract num2 from num1, thus obtaining num1 = 1 and num2 = 4. However, if num1 = 4 and num2 = 5, after one operation, num1 = 4 and num2 = 1.

Return the number of operations required to make either num1 = 0 or num2 = 0.

Example 1:

Input: num1 = 2, num2 = 3
Output: 3
Explanation: 
- Operation 1: num1 = 2, num2 = 3. Since num1 < num2, we subtract num1 from num2 and get num1 = 2, num2 = 3 - 2 = 1.
- Operation 2: num1 = 2, num2 = 1. Since num1 > num2, we subtract num2 from num1.
- Operation 3: num1 = 1, num2 = 1. Since num1 == num2, we subtract num2 from num1.
Now num1 = 0 and num2 = 1. Since num1 == 0, we do not need to perform any further operations.
So the total number of operations required is 3.

Example 2:

Input: num1 = 10, num2 = 10
Output: 1
Explanation: 
- Operation 1: num1 = 10, num2 = 10. Since num1 == num2, we subtract num2 from num1 and get num1 = 10 - 10 = 0.
Now num1 = 0 and num2 = 10. Since num1 == 0, we are done.
So the total number of operations required is 1.

Constraints:

• 0 <= num1, num2 <= 10^5

2025-11-10
3542. Minimum Operations to Convert All Elements to Zero

Topic: Array, Hash Table, Stack, Greedy, Monotonic Stack
Difficulty: Medium

Problem:
You are given an array nums of size n, consisting of non-negative integers. Your task is to apply some (possibly zero) operations on the array so that all elements become 0.

In one operation, you can select a subarray [i, j] (where 0 <= i <= j < n) and set all occurrences of the minimum non-negative integer in that subarray to 0.

Return the minimum number of operations required to make all elements in the array 0.

Example 1:

Input: nums = 0,2

Output: 1

Explanation:

• Select the subarray [1,1] (which is [2]), where the minimum non-negative integer is 2. Setting all occurrences of 2 to 0 results in [0,0].
• Thus, the minimum number of operations required is 1.

Example 2:

Input: nums = 3,1,2,1

Output: 3

Explanation:

• Select subarray [1,3] (which is [1,2,1]), where the minimum non-negative integer is 1. Setting all occurrences of 1 to 0 results in [3,0,2,0].
• Select subarray [2,2] (which is [2]), where the minimum non-negative integer is 2. Setting all occurrences of 2 to 0 results in [3,0,0,0].
• Select subarray [0,0] (which is [3]), where the minimum non-negative integer is 3. Setting all occurrences of 3 to 0 results in [0,0,0,0].
• Thus, the minimum number of operations required is 3.

Example 3:

Input: nums = 1,2,1,2,1,2

Output: 4

Explanation:

• Select subarray [0,5] (which is [1,2,1,2,1,2]), where the minimum non-negative integer is 1. Setting all occurrences of 1 to 0 results in [0,2,0,2,0,2].
• Select subarray [1,1] (which is [2]), where the minimum non-negative integer is 2. Setting all occurrences of 2 to 0 results in [0,0,0,2,0,2].
• Select subarray [3,3] (which is [2]), where the minimum non-negative integer is 2. Setting all occurrences of 2 to 0 results in [0,0,0,0,0,2].
• Select subarray [5,5] (which is [2]), where the minimum non-negative integer is 2. Setting all occurrences of 2 to 0 results in [0,0,0,0,0,0].
• Thus, the minimum number of operations required is 4.

Constraints:

• 1 <= n == nums.length <= 10^5
• 0 <= nums[i] <= 10^5

2025-11-11
474. Ones and Zeroes

Topic: Array, String, Dynamic Programming
Difficulty: Medium

Problem:
You are given an array of binary strings strs and two integers m and n.

Return the size of the largest subset of strs such that there are at most m 0's and n 1's in the subset.

A set x is a subset of a set y if all elements of x are also elements of y.

Example 1:

Input: strs = ["10","0001","111001","1","0"], m = 5, n = 3
Output: 4
Explanation: The largest subset with at most 5 0's and 3 1's is {"10", "0001", "1", "0"}, so the answer is 4.
Other valid but smaller subsets include {"0001", "1"} and {"10", "1", "0"}.
{"111001"} is an invalid subset because it contains 4 1's, greater than the maximum of 3.

Example 2:

Input: strs = ["10","0","1"], m = 1, n = 1
Output: 2
Explanation: The largest subset is {"0", "1"}, so the answer is 2.

Constraints:

• 1 <= strs.length <= 600
• 1 <= strs[i].length <= 100
• strs[i] consists only of digits '0' and '1'.
• 1 <= m, n <= 100

2025-11-12
2654. Minimum Number of Operations to Make All Array Elements Equal to 1

Topic: Array, Math, Number Theory
Difficulty: Medium

Problem:
You are given a 0-indexed array nums consisiting of positive integers. You can do the following operation on the array any number of times:

• Select an index i such that 0 <= i < n - 1 and replace either of nums[i] or nums[i+1] with their gcd value.

Return the minimum number of operations to make all elements of nums equal to 1. If it is impossible, return -1.

The gcd of two integers is the greatest common divisor of the two integers.

Example 1:

Input: nums = [2,6,3,4]
Output: 4
Explanation: We can do the following operations:
- Choose index i = 2 and replace nums[2] with gcd(3,4) = 1. Now we have nums = [2,6,1,4].
- Choose index i = 1 and replace nums[1] with gcd(6,1) = 1. Now we have nums = [2,1,1,4].
- Choose index i = 0 and replace nums[0] with gcd(2,1) = 1. Now we have nums = [1,1,1,4].
- Choose index i = 2 and replace nums[3] with gcd(1,4) = 1. Now we have nums = [1,1,1,1].

Example 2:

Input: nums = [2,10,6,14]
Output: -1
Explanation: It can be shown that it is impossible to make all the elements equal to 1.

Constraints:

• 2 <= nums.length <= 50
• 1 <= nums[i] <= 10^6

2025-11-13
3228. Maximum Number of Operations to Move Ones to the End

Topic: String, Greedy, Counting
Difficulty: Medium

Problem:
You are given a binary string s.

You can perform the following operation on the string any number of times:

• Choose any index i from the string where i + 1 < s.length such that s[i] == '1' and s[i + 1] == '0'.
• Move the character s[i] to the right until it reaches the end of the string or another '1'. For example, for s = "010010", if we choose i = 1, the resulting string will be s = "000110".

Return the maximum number of operations that you can perform.

Example 1:

Input: s = "1001101"

Output: 4

Explanation:

We can perform the following operations:

• Choose index i = 0. The resulting string is s = "0011101".
• Choose index i = 4. The resulting string is s = "0011011".
• Choose index i = 3. The resulting string is s = "0010111".
• Choose index i = 2. The resulting string is s = "0001111".

Example 2:

Input: s = "00111"

Output: 0

Constraints:

• 1 <= s.length <= 10^5
• s[i] is either '0' or '1'.

2025-11-14
2536. Increment Submatrices by One

Topic: Array, Matrix, Prefix Sum
Difficulty: Medium

Problem:
You are given a positive integer n, indicating that we initially have an n x n 0-indexed integer matrix mat filled with zeroes.

You are also given a 2D integer array query. For each query[i] = [row1_i, col1_i, row2_i, col2_i], you should do the following operation:

• Add 1 to every element in the submatrix with the top left corner (row1_i, col1_i) and the bottom right corner (row2_i, col2_i). That is, add 1 to mat[x][y] for all row1_i <= x <= row2_i and col1_i <= y <= col2_i.

Return the matrix mat after performing every query.

Example 1:

Image: https://assets.leetcode.com/uploads/2022/11/24/p2example11.png

Input: n = 3, queries = [[1,1,2,2],[0,0,1,1]]
Output: [[1,1,0],[1,2,1],[0,1,1]]
Explanation: The diagram above shows the initial matrix, the matrix after the first query, and the matrix after the second query.
- In the first query, we add 1 to every element in the submatrix with the top left corner (1, 1) and bottom right corner (2, 2).
- In the second query, we add 1 to every element in the submatrix with the top left corner (0, 0) and bottom right corner (1, 1).

Example 2:

Image: https://assets.leetcode.com/uploads/2022/11/24/p2example22.png

Input: n = 2, queries = [[0,0,1,1]]
Output: [[1,1],[1,1]]
Explanation: The diagram above shows the initial matrix and the matrix after the first query.
- In the first query we add 1 to every element in the matrix.

Constraints:

• 1 <= n <= 500
• 1 <= queries.length <= 10^4
• 0 <= row1_i <= row2_i < n
• 0 <= col1_i <= col2_i < n

2025-11-15
3234. Count the Number of Substrings With Dominant Ones

Topic: String, Sliding Window, Enumeration
Difficulty: Medium

Problem:
You are given a binary string s.

Return the number of substrings with dominant ones.

A string has dominant ones if the number of ones in the string is greater than or equal to the square of the number of zeros in the string.

Example 1:

Input: s = "00011"

Output: 5

Explanation:

The substrings with dominant ones are shown in the table below.

ijsi..jNumber of ZerosNumber of Ones33101441012301113411022401112
Example 2:

Input: s = "101101"

Output: 16

Explanation:

The substrings with non-dominant ones are shown in the table below.

Since there are 21 substrings total and 5 of them have non-dominant ones, it follows that there are 16 substrings with dominant ones.

ijsi..jNumber of ZerosNumber of Ones110104401014011022041011023150110123

Constraints:

• 1 <= s.length <= 4 * 10^4
• s consists only of characters '0' and '1'.

2025-11-16
1513. Number of Substrings With Only 1s

Topic: Math, String
Difficulty: Medium

Problem:
Given a binary string s, return the number of substrings with all characters 1's. Since the answer may be too large, return it modulo 10^9 + 7.

Example 1:

Input: s = "0110111"
Output: 9
Explanation: There are 9 substring in total with only 1's characters.
"1" -> 5 times.
"11" -> 3 times.
"111" -> 1 time.

Example 2:

Input: s = "101"
Output: 2
Explanation: Substring "1" is shown 2 times in s.

Example 3:

Input: s = "111111"
Output: 21
Explanation: Each substring contains only 1's characters.

Constraints:

• 1 <= s.length <= 10^5
• s[i] is either '0' or '1'.

2025-11-17
1437. Check If All 1's Are at Least Length K Places Away

Topic: Array
Difficulty: Easy

Problem:
Given an binary array nums and an integer k, return true if all 1's are at least k places away from each other, otherwise return false.

Example 1:

Image: https://assets.leetcode.com/uploads/2020/04/15/sample_1_1791.png

Input: nums = [1,0,0,0,1,0,0,1], k = 2
Output: true
Explanation: Each of the 1s are at least 2 places away from each other.

Example 2:

Image: https://assets.leetcode.com/uploads/2020/04/15/sample_2_1791.png

Input: nums = [1,0,0,1,0,1], k = 2
Output: false
Explanation: The second 1 and third 1 are only one apart from each other.

Constraints:

• 1 <= nums.length <= 10^5
• 0 <= k <= nums.length
• nums[i] is 0 or 1

2025-11-18
717. 1-bit and 2-bit Characters

Topic: Array
Difficulty: Easy

Problem:
We have two special characters:

• The first character can be represented by one bit 0.
• The second character can be represented by two bits (10 or 11).

Given a binary array bits that ends with 0, return true if the last character must be a one-bit character.

Example 1:

Input: bits = [1,0,0]
Output: true
Explanation: The only way to decode it is two-bit character and one-bit character.
So the last character is one-bit character.

Example 2:

Input: bits = [1,1,1,0]
Output: false
Explanation: The only way to decode it is two-bit character and two-bit character.
So the last character is not one-bit character.

Constraints:

• 1 <= bits.length <= 1000
• bits[i] is either 0 or 1.

2025-11-19
2154. Keep Multiplying Found Values by Two

Topic: Array, Hash Table, Sorting, Simulation
Difficulty: Easy

Problem:
You are given an array of integers nums. You are also given an integer original which is the first number that needs to be searched for in nums.

You then do the following steps:

1. If original is found in nums, multiply it by two (i.e., set original = 2 * original).
2. Otherwise, stop the process.
3. Repeat this process with the new number as long as you keep finding the number.

Return the final value of original.

Example 1:

Input: nums = [5,3,6,1,12], original = 3
Output: 24
Explanation: 
- 3 is found in nums. 3 is multiplied by 2 to obtain 6.
- 6 is found in nums. 6 is multiplied by 2 to obtain 12.
- 12 is found in nums. 12 is multiplied by 2 to obtain 24.
- 24 is not found in nums. Thus, 24 is returned.

Example 2:

Input: nums = [2,7,9], original = 4
Output: 4
Explanation:
- 4 is not found in nums. Thus, 4 is returned.

Constraints:

• 1 <= nums.length <= 1000
• 1 <= nums[i], original <= 1000