2025-10-27
2125. Number of Laser Beams in a Bank

Topic: Array, Math, String, Matrix
Difficulty: Medium

Problem:
Anti-theft security devices are activated inside a bank. You are given a 0-indexed binary string array bank representing the floor plan of the bank, which is an m x n 2D matrix. bank[i] represents the i^th row, consisting of '0's and '1's. '0' means the cell is empty, while'1' means the cell has a security device.

There is one laser beam between any two security devices if both conditions are met:

• The two devices are located on two different rows: r_1 and r_2, where r_1 < r_2.
• For each row i where r_1 < i < r_2, there are no security devices in the i^th row.

Laser beams are independent, i.e., one beam does not interfere nor join with another.

Return the total number of laser beams in the bank.

Example 1:

Image: https://assets.leetcode.com/uploads/2021/12/24/laser1.jpg

Input: bank = ["011001","000000","010100","001000"]
Output: 8
Explanation: Between each of the following device pairs, there is one beam. In total, there are 8 beams:
 * bank[0][1] -- bank[2][1]
 * bank[0][1] -- bank[2][3]
 * bank[0][2] -- bank[2][1]
 * bank[0][2] -- bank[2][3]
 * bank[0][5] -- bank[2][1]
 * bank[0][5] -- bank[2][3]
 * bank[2][1] -- bank[3][2]
 * bank[2][3] -- bank[3][2]
Note that there is no beam between any device on the 0^th row with any on the 3^rd row.
This is because the 2^nd row contains security devices, which breaks the second condition.

Example 2:

Image: https://assets.leetcode.com/uploads/2021/12/24/laser2.jpg

Input: bank = ["000","111","000"]
Output: 0
Explanation: There does not exist two devices located on two different rows.

Constraints:

• m == bank.length
• n == bank[i].length
• 1 <= m, n <= 500
• bank[i][j] is either '0' or '1'.

2025-10-28
3354. Make Array Elements Equal to Zero

Topic: Array, Simulation, Prefix Sum
Difficulty: Easy

Problem:
You are given an integer array nums.

Start by selecting a starting position curr such that nums[curr] == 0, and choose a movement direction of either left or right.

After that, you repeat the following process:

• If curr is out of the range [0, n - 1], this process ends.
• If nums[curr] == 0, move in the current direction by incrementing curr if you are moving right, or decrementing curr if you are moving left.
• Else if nums[curr] > 0:
• Decrement nums[curr] by 1.
• Reverse your movement direction (left becomes right and vice versa).
• Take a step in your new direction.

A selection of the initial position curr and movement direction is considered valid if every element in nums becomes 0 by the end of the process.

Return the number of possible valid selections.

Example 1:

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

Output: 2

Explanation:

The only possible valid selections are the following:

• Choose curr = 3, and a movement direction to the left.
• [1,0,2,0,3] -> [1,0,2,0,3] -> [1,0,1,0,3] -> [1,0,1,0,3] -> [1,0,1,0,2] -> [1,0,1,0,2] -> [1,0,0,0,2] -> [1,0,0,0,2] -> [1,0,0,0,1] -> [1,0,0,0,1] -> [1,0,0,0,1] -> [1,0,0,0,1] -> [0,0,0,0,1] -> [0,0,0,0,1] -> [0,0,0,0,1] -> [0,0,0,0,1] -> [0,0,0,0,0].
• Choose curr = 3, and a movement direction to the right.
• [1,0,2,0,3] -> [1,0,2,0,3] -> [1,0,2,0,2] -> [1,0,2,0,2] -> [1,0,1,0,2] -> [1,0,1,0,2] -> [1,0,1,0,1] -> [1,0,1,0,1] -> [1,0,0,0,1] -> [1,0,0,0,1] -> [1,0,0,0,0] -> [1,0,0,0,0] -> [1,0,0,0,0] -> [1,0,0,0,0] -> [0,0,0,0,0].

Example 2:

Input: nums = 2,3,4,0,4,1,0

Output: 0

Explanation:

There are no possible valid selections.

Constraints:

• 1 <= nums.length <= 100
• 0 <= nums[i] <= 100
• There is at least one element i where nums[i] == 0.

2025-10-29
3370. Smallest Number With All Set Bits

Topic: Math, Bit Manipulation
Difficulty: Easy

Problem:
You are given a positive number n.

Return the smallest number x greater than or equal to n, such that the binary representation of x contains only set bits

Example 1:

Input: n = 5

Output: 7

Explanation:

The binary representation of 7 is "111".

Example 2:

Input: n = 10

Output: 15

Explanation:

The binary representation of 15 is "1111".

Example 3:

Input: n = 3

Output: 3

Explanation:

The binary representation of 3 is "11".

Constraints:

• 1 <= n <= 1000

2025-10-30
1526. Minimum Number of Increments on Subarrays to Form a Target Array

Topic: Array, Dynamic Programming, Stack, Greedy, Monotonic Stack
Difficulty: Hard

Problem:
You are given an integer array target. You have an integer array initial of the same size as target with all elements initially zeros.

In one operation you can choose any subarray from initial and increment each value by one.

Return the minimum number of operations to form a target array from initial.

The test cases are generated so that the answer fits in a 32-bit integer.

Example 1:

Input: target = [1,2,3,2,1]
Output: 3
Explanation: We need at least 3 operations to form the target array from the initial array.
[0,0,0,0,0] increment 1 from index 0 to 4 (inclusive).
[1,1,1,1,1] increment 1 from index 1 to 3 (inclusive).
[1,2,2,2,1] increment 1 at index 2.
[1,2,3,2,1] target array is formed.

Example 2:

Input: target = [3,1,1,2]
Output: 4
Explanation: [0,0,0,0] -> [1,1,1,1] -> [1,1,1,2] -> [2,1,1,2] -> [3,1,1,2]

Example 3:

Input: target = [3,1,5,4,2]
Output: 7
Explanation: [0,0,0,0,0] -> [1,1,1,1,1] -> [2,1,1,1,1] -> [3,1,1,1,1] -> [3,1,2,2,2] -> [3,1,3,3,2] -> [3,1,4,4,2] -> [3,1,5,4,2].

Constraints:

• 1 <= target.length <= 10^5
• 1 <= target[i] <= 10^5

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.