2024-02-26
100. Same Tree

Topic: Tree, Depth-First Search, Breadth-First Search, Binary Tree
Difficulty: Easy

Problem:
Given the roots of two binary trees p and q, write a function to check if they are the same or not.

Two binary trees are considered the same if they are structurally identical, and the nodes have the same value.

Example 1:

Image: https://assets.leetcode.com/uploads/2020/12/20/ex1.jpg

Input: p = [1,2,3], q = [1,2,3]
Output: true

Example 2:

Image: https://assets.leetcode.com/uploads/2020/12/20/ex2.jpg

Input: p = [1,2], q = [1,null,2]
Output: false

Example 3:

Image: https://assets.leetcode.com/uploads/2020/12/20/ex3.jpg

Input: p = [1,2,1], q = [1,1,2]
Output: false

Constraints:

• The number of nodes in both trees is in the range [0, 100].
• -10^4 <= Node.val <= 10^4

2024-02-27
543. Diameter of Binary Tree

Topic: Tree, Depth-First Search, Binary Tree
Difficulty: Easy

Problem:
Given the root of a binary tree, return the length of the diameter of the tree.

The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

The length of a path between two nodes is represented by the number of edges between them.

Example 1:

Image: https://assets.leetcode.com/uploads/2021/03/06/diamtree.jpg

Input: root = [1,2,3,4,5]
Output: 3
Explanation: 3 is the length of the path [4,2,1,3] or [5,2,1,3].

Example 2:

Input: root = [1,2]
Output: 1

Constraints:

• The number of nodes in the tree is in the range [1, 10^4].
• -100 <= Node.val <= 100

2024-02-28
513. Find Bottom Left Tree Value

Topic: Tree, Depth-First Search, Breadth-First Search, Binary Tree
Difficulty: Medium

Problem:
Given the root of a binary tree, return the leftmost value in the last row of the tree.

Example 1:

Image: https://assets.leetcode.com/uploads/2020/12/14/tree1.jpg

Input: root = [2,1,3]
Output: 1

Example 2:

Image: https://assets.leetcode.com/uploads/2020/12/14/tree2.jpg

Input: root = [1,2,3,4,null,5,6,null,null,7]
Output: 7

Constraints:

• The number of nodes in the tree is in the range [1, 10^4].
• -2^31 <= Node.val <= 2^31 - 1

2024-02-29
1609. Even Odd Tree

Topic: Tree, Breadth-First Search, Binary Tree
Difficulty: Medium

Problem:
A binary tree is named Even-Odd if it meets the following conditions:

• The root of the binary tree is at level index 0, its children are at level index 1, their children are at level index 2, etc.
• For every even-indexed level, all nodes at the level have odd integer values in strictly increasing order (from left to right).
• For every odd-indexed level, all nodes at the level have even integer values in strictly decreasing order (from left to right).

Given the root of a binary tree, return true if the binary tree is Even-Odd, otherwise return false.

Example 1:

Image: https://assets.leetcode.com/uploads/2020/09/15/sample_1_1966.png

Input: root = [1,10,4,3,null,7,9,12,8,6,null,null,2]
Output: true
Explanation: The node values on each level are:
Level 0: [1]
Level 1: [10,4]
Level 2: [3,7,9]
Level 3: [12,8,6,2]
Since levels 0 and 2 are all odd and increasing and levels 1 and 3 are all even and decreasing, the tree is Even-Odd.

Example 2:

Image: https://assets.leetcode.com/uploads/2020/09/15/sample_2_1966.png

Input: root = [5,4,2,3,3,7]
Output: false
Explanation: The node values on each level are:
Level 0: [5]
Level 1: [4,2]
Level 2: [3,3,7]
Node values in level 2 must be in strictly increasing order, so the tree is not Even-Odd.

Example 3:

Image: https://assets.leetcode.com/uploads/2020/09/22/sample_1_333_1966.png

Input: root = [5,9,1,3,5,7]
Output: false
Explanation: Node values in the level 1 should be even integers.

Constraints:

• The number of nodes in the tree is in the range [1, 10^5].
• 1 <= Node.val <= 10^6

2024-03-01
2864. Maximum Odd Binary Number

Topic: Math, String, Greedy
Difficulty: Easy

Problem:
You are given a binary string s that contains at least one '1'.

You have to rearrange the bits in such a way that the resulting binary number is the maximum odd binary number that can be created from this combination.

Return a string representing the maximum odd binary number that can be created from the given combination.

Note that the resulting string can have leading zeros.

Example 1:

Input: s = "010"
Output: "001"
Explanation: Because there is just one '1', it must be in the last position. So the answer is "001".

Example 2:

Input: s = "0101"
Output: "1001"
Explanation: One of the '1's must be in the last position. The maximum number that can be made with the remaining digits is "100". So the answer is "1001".

Constraints:

• 1 <= s.length <= 100
• s consists only of '0' and '1'.
• s contains at least one '1'.

2024-03-02
977. Squares of a Sorted Array

Topic: Array, Two Pointers, Sorting
Difficulty: Easy

Problem:
Given an integer array nums sorted in non-decreasing order, return an array of the squares of each number sorted in non-decreasing order.

Example 1:

Input: nums = [-4,-1,0,3,10]
Output: [0,1,9,16,100]
Explanation: After squaring, the array becomes [16,1,0,9,100].
After sorting, it becomes [0,1,9,16,100].

Example 2:

Input: nums = [-7,-3,2,3,11]
Output: [4,9,9,49,121]

Constraints:

• 1 <= nums.length <= 10^4
• -10^4 <= nums[i] <= 10^4
• nums is sorted in non-decreasing order.

Follow up: Squaring each element and sorting the new array is very trivial, could you find an O(n) solution using a different approach?

2024-03-03
19. Remove Nth Node From End of List

Topic: Linked List, Two Pointers
Difficulty: Medium

Problem:
Given the head of a linked list, remove the n^th node from the end of the list and return its head.

Example 1:

Image: https://assets.leetcode.com/uploads/2020/10/03/remove_ex1.jpg

Input: head = [1,2,3,4,5], n = 2
Output: [1,2,3,5]

Example 2:

Input: head = [1], n = 1
Output: []

Example 3:

Input: head = [1,2], n = 1
Output: [1]

Constraints:

• The number of nodes in the list is sz.
• 1 <= sz <= 30
• 0 <= Node.val <= 100
• 1 <= n <= sz

Follow up: Could you do this in one pass?

2024-03-05
1750. Minimum Length of String After Deleting Similar Ends

Topic: Two Pointers, String
Difficulty: Medium

Problem:
Given a string s consisting only of characters 'a', 'b', and 'c'. You are asked to apply the following algorithm on the string any number of times:

1. Pick a non-empty prefix from the string s where all the characters in the prefix are equal.
2. Pick a non-empty suffix from the string s where all the characters in this suffix are equal.
3. The prefix and the suffix should not intersect at any index.
4. The characters from the prefix and suffix must be the same.
5. Delete both the prefix and the suffix.

Return the minimum length of s after performing the above operation any number of times (possibly zero times).

Example 1:

Input: s = "ca"
Output: 2
Explanation: You can't remove any characters, so the string stays as is.

Example 2:

Input: s = "cabaabac"
Output: 0
Explanation: An optimal sequence of operations is:
- Take prefix = "c" and suffix = "c" and remove them, s = "abaaba".
- Take prefix = "a" and suffix = "a" and remove them, s = "baab".
- Take prefix = "b" and suffix = "b" and remove them, s = "aa".
- Take prefix = "a" and suffix = "a" and remove them, s = "".

Example 3:

Input: s = "aabccabba"
Output: 3
Explanation: An optimal sequence of operations is:
- Take prefix = "aa" and suffix = "a" and remove them, s = "bccabb".
- Take prefix = "b" and suffix = "bb" and remove them, s = "cca".

Constraints:

• 1 <= s.length <= 10^5
• s only consists of characters 'a', 'b', and 'c'.

2024-03-06
141. Linked List Cycle

Topic: Hash Table, Linked List, Two Pointers
Difficulty: Easy

Problem:
Given head, the head of a linked list, determine if the linked list has a cycle in it.

There is a cycle in a linked list if there is some node in the list that can be reached again by continuously following the next pointer. Internally, pos is used to denote the index of the node that tail's next pointer is connected to. Note that pos is not passed as a parameter.

Return true if there is a cycle in the linked list. Otherwise, return false.

Example 1:

Image: https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist.png

Input: head = [3,2,0,-4], pos = 1
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 1st node (0-indexed).

Example 2:

Image: https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test2.png

Input: head = [1,2], pos = 0
Output: true
Explanation: There is a cycle in the linked list, where the tail connects to the 0th node.

Example 3:

Image: https://assets.leetcode.com/uploads/2018/12/07/circularlinkedlist_test3.png

Input: head = [1], pos = -1
Output: false
Explanation: There is no cycle in the linked list.

Constraints:

• The number of the nodes in the list is in the range [0, 10^4].
• -10^5 <= Node.val <= 10^5
• pos is -1 or a valid index in the linked-list.

Follow up: Can you solve it using O(1) (i.e. constant) memory?

2024-03-12
1171. Remove Zero Sum Consecutive Nodes from Linked List

Topic: Hash Table, Linked List
Difficulty: Medium

Problem:
Given the head of a linked list, we repeatedly delete consecutive sequences of nodes that sum to 0 until there are no such sequences.

After doing so, return the head of the final linked list.  You may return any such answer.

(Note that in the examples below, all sequences are serializations of ListNode objects.)

Example 1:

Input: head = [1,2,-3,3,1]
Output: [3,1]
Note: The answer [1,2,1] would also be accepted.

Example 2:

Input: head = [1,2,3,-3,4]
Output: [1,2,4]

Example 3:

Input: head = [1,2,3,-3,-2]
Output: [1]

Constraints:

• The given linked list will contain between 1 and 1000 nodes.
• Each node in the linked list has -1000 <= node.val <= 1000.

2024-03-13
2485. Find the Pivot Integer

Topic: Math, Prefix Sum
Difficulty: Easy

Problem:
Given a positive integer n, find the pivot integer x such that:

• The sum of all elements between 1 and x inclusively equals the sum of all elements between x and n inclusively.

Return the pivot integer x. If no such integer exists, return -1. It is guaranteed that there will be at most one pivot index for the given input.

Example 1:

Input: n = 8
Output: 6
Explanation: 6 is the pivot integer since: 1 + 2 + 3 + 4 + 5 + 6 = 6 + 7 + 8 = 21.

Example 2:

Input: n = 1
Output: 1
Explanation: 1 is the pivot integer since: 1 = 1.

Example 3:

Input: n = 4
Output: -1
Explanation: It can be proved that no such integer exist.

Constraints:

• 1 <= n <= 1000

2024-03-14
930. Binary Subarrays With Sum

Topic: Array, Hash Table, Sliding Window, Prefix Sum
Difficulty: Medium

Problem:
Given a binary array nums and an integer goal, return the number of non-empty subarrays with a sum goal.

A subarray is a contiguous part of the array.

Example 1:

Input: nums = [1,0,1,0,1], goal = 2
Output: 4
Explanation: The 4 subarrays are bolded and underlined below:
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]
[1,0,1,0,1]

Example 2:

Input: nums = [0,0,0,0,0], goal = 0
Output: 15

Constraints:

• 1 <= nums.length <= 3 * 10^4
• nums[i] is either 0 or 1.
• 0 <= goal <= nums.length

2024-03-15
238. Product of Array Except Self

Topic: Array, Prefix Sum
Difficulty: Medium

Problem:
Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i].

The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

You must write an algorithm that runs in O(n) time and without using the division operation.

Example 1:

Input: nums = [1,2,3,4]
Output: [24,12,8,6]

Example 2:

Input: nums = [-1,1,0,-3,3]
Output: [0,0,9,0,0]

Constraints:

• 2 <= nums.length <= 10^5
• -30 <= nums[i] <= 30
• The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

Follow up: Can you solve the problem in O(1) extra space complexity? (The output array does not count as extra space for space complexity analysis.)

2024-03-16
525. Contiguous Array

Topic: Array, Hash Table, Prefix Sum
Difficulty: Medium

Problem:
Given a binary array nums, return the maximum length of a contiguous subarray with an equal number of 0 and 1.

Example 1:

Input: nums = [0,1]
Output: 2
Explanation: [0, 1] is the longest contiguous subarray with an equal number of 0 and 1.

Example 2:

Input: nums = [0,1,0]
Output: 2
Explanation: [0, 1] (or [1, 0]) is a longest contiguous subarray with equal number of 0 and 1.

Constraints:

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

2024-03-17
57. Insert Interval

Topic: Array
Difficulty: Medium

Problem:
You are given an array of non-overlapping intervals intervals where intervals[i] = [start_i, end_i] represent the start and the end of the i^th interval and intervals is sorted in ascending order by start_i. You are also given an interval newInterval = [start, end] that represents the start and end of another interval.

Insert newInterval into intervals such that intervals is still sorted in ascending order by start_i and intervals still does not have any overlapping intervals (merge overlapping intervals if necessary).

Return intervals after the insertion.

Note that you don't need to modify intervals in-place. You can make a new array and return it.

Example 1:

Input: intervals = [[1,3],[6,9]], newInterval = [2,5]
Output: [[1,5],[6,9]]

Example 2:

Input: intervals = [[1,2],[3,5],[6,7],[8,10],[12,16]], newInterval = [4,8]
Output: [[1,2],[3,10],[12,16]]
Explanation: Because the new interval [4,8] overlaps with [3,5],[6,7],[8,10].

Constraints:

• 0 <= intervals.length <= 10^4
• intervals[i].length == 2
• 0 <= start_i <= end_i <= 10^5
• intervals is sorted by start_i in ascending order.
• newInterval.length == 2
• 0 <= start <= end <= 10^5

2024-03-18
452. Minimum Number of Arrows to Burst Balloons

Topic: Array, Greedy, Sorting
Difficulty: Medium

Problem:
There are some spherical balloons taped onto a flat wall that represents the XY-plane. The balloons are represented as a 2D integer array points where points[i] = [x_start, x_end] denotes a balloon whose horizontal diameter stretches between x_start and x_end. You do not know the exact y-coordinates of the balloons.

Arrows can be shot up directly vertically (in the positive y-direction) from different points along the x-axis. A balloon with x_start and x_end is burst by an arrow shot at x if x_start <= x <= x_end. There is no limit to the number of arrows that can be shot. A shot arrow keeps traveling up infinitely, bursting any balloons in its path.

Given the array points, return the minimum number of arrows that must be shot to burst all balloons.

Example 1:

Input: points = [[10,16],[2,8],[1,6],[7,12]]
Output: 2
Explanation: The balloons can be burst by 2 arrows:
- Shoot an arrow at x = 6, bursting the balloons [2,8] and [1,6].
- Shoot an arrow at x = 11, bursting the balloons [10,16] and [7,12].

Example 2:

Input: points = [[1,2],[3,4],[5,6],[7,8]]
Output: 4
Explanation: One arrow needs to be shot for each balloon for a total of 4 arrows.

Example 3:

Input: points = [[1,2],[2,3],[3,4],[4,5]]
Output: 2
Explanation: The balloons can be burst by 2 arrows:
- Shoot an arrow at x = 2, bursting the balloons [1,2] and [2,3].
- Shoot an arrow at x = 4, bursting the balloons [3,4] and [4,5].

Constraints:

• 1 <= points.length <= 10^5
• points[i].length == 2
• -2^31 <= x_start < x_end <= 2^31 - 1

2024-03-19
621. Task Scheduler

Topic: Array, Hash Table, Greedy, Sorting, Heap (Priority Queue), Counting
Difficulty: Medium

Problem:
You are given an array of CPU tasks, each represented by letters A to Z, and a cooling time, n. Each cycle or interval allows the completion of one task. Tasks can be completed in any order, but there's a constraint: identical tasks must be separated by at least n intervals due to cooling time.

​Return the minimum number of intervals required to complete all tasks.

Example 1:

Input: tasks = "A","A","A","B","B","B", n = 2

Output: 8

Explanation: A possible sequence is: A -> B -> idle -> A -> B -> idle -> A -> B.

After completing task A, you must wait two cycles before doing A again. The same applies to task B. In the 3^rd interval, neither A nor B can be done, so you idle. By the 4^th cycle, you can do A again as 2 intervals have passed.

Example 2:

Input: tasks = "A","C","A","B","D","B", n = 1

Output: 6

Explanation: A possible sequence is: A -> B -> C -> D -> A -> B.

With a cooling interval of 1, you can repeat a task after just one other task.

Example 3:

Input: tasks = "A","A","A", "B","B","B", n = 3

Output: 10

Explanation: A possible sequence is: A -> B -> idle -> idle -> A -> B -> idle -> idle -> A -> B.

There are only two types of tasks, A and B, which need to be separated by 3 intervals. This leads to idling twice between repetitions of these tasks.

Constraints:

• 1 <= tasks.length <= 10^4
• tasks[i] is an uppercase English letter.
• 0 <= n <= 100

2024-03-20
1669. Merge In Between Linked Lists

Topic: Linked List
Difficulty: Medium

Problem:
You are given two linked lists: list1 and list2 of sizes n and m respectively.

Remove list1's nodes from the a^th node to the b^th node, and put list2 in their place.

The blue edges and nodes in the following figure indicate the result:

Image: https://assets.leetcode.com/uploads/2020/11/05/fig1.png

Build the result list and return its head.

Example 1:

Image: https://assets.leetcode.com/uploads/2024/03/01/ll.png

Input: list1 = [10,1,13,6,9,5], a = 3, b = 4, list2 = [1000000,1000001,1000002]
Output: [10,1,13,1000000,1000001,1000002,5]
Explanation: We remove the nodes 3 and 4 and put the entire list2 in their place. The blue edges and nodes in the above figure indicate the result.

Example 2:

Image: https://assets.leetcode.com/uploads/2020/11/05/merge_linked_list_ex2.png

Input: list1 = [0,1,2,3,4,5,6], a = 2, b = 5, list2 = [1000000,1000001,1000002,1000003,1000004]
Output: [0,1,1000000,1000001,1000002,1000003,1000004,6]
Explanation: The blue edges and nodes in the above figure indicate the result.

Constraints:

• 3 <= list1.length <= 10^4
• 1 <= a <= b < list1.length - 1
• 1 <= list2.length <= 10^4

2024-03-21
206. Reverse Linked List

Topic: Linked List, Recursion
Difficulty: Easy

Problem:
Given the head of a singly linked list, reverse the list, and return the reversed list.

Example 1:

Image: https://assets.leetcode.com/uploads/2021/02/19/rev1ex1.jpg

Input: head = [1,2,3,4,5]
Output: [5,4,3,2,1]

Example 2:

Image: https://assets.leetcode.com/uploads/2021/02/19/rev1ex2.jpg

Input: head = [1,2]
Output: [2,1]

Example 3:

Input: head = []
Output: []

Constraints:

• The number of nodes in the list is the range [0, 5000].
• -5000 <= Node.val <= 5000

Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both?

2024-03-22
234. Palindrome Linked List

Topic: Linked List, Two Pointers, Stack, Recursion
Difficulty: Easy

Problem:
Given the head of a singly linked list, return true if it is a palindrome or false otherwise.

Example 1:

Image: https://assets.leetcode.com/uploads/2021/03/03/pal1linked-list.jpg

Input: head = [1,2,2,1]
Output: true

Example 2:

Image: https://assets.leetcode.com/uploads/2021/03/03/pal2linked-list.jpg

Input: head = [1,2]
Output: false

Constraints:

• The number of nodes in the list is in the range [1, 10^5].
• 0 <= Node.val <= 9

Follow up: Could you do it in O(n) time and O(1) space?

2024-03-23
143. Reorder List

Topic: Linked List, Two Pointers, Stack, Recursion
Difficulty: Medium

Problem:
You are given the head of a singly linked-list. The list can be represented as:

L_0 → L_1 → … → L_n - 1 → L_n

Reorder the list to be on the following form:

L_0 → L_n → L_1 → L_n - 1 → L_2 → L_n - 2 → …

You may not modify the values in the list's nodes. Only nodes themselves may be changed.

Example 1:

Image: https://assets.leetcode.com/uploads/2021/03/04/reorder1linked-list.jpg

Input: head = [1,2,3,4]
Output: [1,4,2,3]

Example 2:

Image: https://assets.leetcode.com/uploads/2021/03/09/reorder2-linked-list.jpg

Input: head = [1,2,3,4,5]
Output: [1,5,2,4,3]

Constraints:

• The number of nodes in the list is in the range [1, 5 * 10^4].
• 1 <= Node.val <= 1000