Ikwhan Chang
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Month: February 2021

[LC] 21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a sorted list. The list should be made by splicing together the nodes of the first two lists. Example 1: Input: l1 = [1,2,4], l2 = [1,3,4] Output: [1,1,2,3,4,4] Example 2: Input: l1 = [], l2 = [] Output: [] Example 3: Input: l1 = [], l2 = ...

[LC] 253. Meeting Rooms II

Given an array of meeting time intervals intervals where intervals[i] = [starti, endi], return the minimum number of conference rooms required. Example 1: Input: intervals = [[0,30],[5,10],[15,20]] Output: 2 Example 2: Input: intervals = [[7,10],[2,4]] Output: 1 Constraints: 1 <= intervals.length <= 104 0 <= starti < endi <= 106 Approach It is another problem I spent about 5 hours until midnight ...

[LC] 206. Reverse Linked List

Reverse a singly linked list. Example: Input: 1->2->3->4->5->NULL Output: 5->4->3->2->1->NULL Follow up: A linked list can be reversed either iteratively or recursively. Could you implement both? Approach It is another problem I spent about 3 hours though it is in “easy” category. I’ve solved it about 20 times before but every time when I was ...

[LC] 53. Maximum Subarray

Given an integer array nums, find the contiguous subarray (containing at least one number) which has the largest sum and return its sum. Example 1: Input: nums = [-2,1,-3,4,-1,2,1,-5,4] Output: 6 Explanation: [4,-1,2,1] has the largest sum = 6. Example 2: Input: nums = [1] Output: 1 Example 3: Input: nums = [0] Output: 0 Example 4: ...

[LC] 238. Product of Array Except Self

Given an array nums of n integers where n > 1,  return an array output such that output[i] is equal to the product of all the elements of nums except nums[i]. Example: Input: [1,2,3,4] Output: [24,12,8,6] Constraint: It’s guaranteed that the product of the elements of any prefix or suffix of the array (including the whole array) fits in a 32 bit integer. Note: Please solve it without division and in O(n). ...

[LC] 121. Best Time to Buy and Sell Stock

You are given an array prices where prices[i] is the price of a given stock on the ith day. You want to maximize your profit by choosing a single day to buy one stock and choosing a different day in the future to sell that stock. Return the maximum profit you can achieve from this transaction. If you cannot achieve any profit, return 0. Example 1: Input: ...

[LC] 2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order, and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list. You may assume the two numbers do not contain any leading zero, except the number 0 itself. Example 1: Input: ...

[LC] 56. Merge Intervals

Given an array of intervals where intervals[i] = [starti, endi], merge all overlapping intervals, and return an array of the non-overlapping intervals that cover all the intervals in the input. Example 1: Input: intervals = [[1,3],[2,6],[8,10],[15,18]] Output: [[1,6],[8,10],[15,18]] Explanation: Since intervals [1,3] and [2,6] overlaps, merge them into [1,6]. Example 2: Input: intervals = [[1,4],[4,5]] Output: [[1,5]] Explanation: Intervals [1,4] ...

[LC] 42. Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining. Example 1: Input: height = [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6 Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. ...

[LC] 200. Number of Islands

Given an m x n 2d grid map of '1's (land) and '0's (water), return the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water. Approach Simple breath first search (BFS). Nothing special. Just keep looking for non visited grid ...