Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining. For example, given [0,1,0,2,1,0,1,3,2,1,2,1], return 6. Analysis This problem is similar to Candy. It can be solve by scanning from both sides and then get the total. Java … Read more
Given a 2D board and a list of words from the dictionary, find all words in the board. Each word must be constructed from letters of sequentially adjacent cell, where “adjacent” cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once in a word. For example, given … Read more
Given a 2D board and a word, find if the word exists in the grid. The word can be constructed from letters of sequentially adjacent cell, where “adjacent” cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once. For example, given board = [ [“ABCE”], [“SFCS”], [“ADEE”] … Read more
Given a string S, you are allowed to convert it to a palindrome by adding characters in front of it. Find and return the shortest palindrome you can find by performing this transformation. For example, given “aacecaaa”, return “aaacecaaa”; given “abcd”, return “dcbabcd”. Java Solution 1 public String shortestPalindrome(String s) { int i=0; int j=s.length()-1; … Read more
Given an unsorted integer array, find the first missing positive integer. For example, given [1,2,0] return 3 and [3,4,-1,1] return 2. Your algorithm should run in O(n) time and uses constant space. Analysis This problem can solve by using a bucket-sort like algorithm. Let’s consider finding first missing positive and 0 first. The key fact … Read more
Given an array of integers and an integer k, return true if and only if there are two distinct indices i and j in the array such that nums[i] = nums[j] and the difference between i and j is at most k. Java Solution 1 – HashMap public boolean containsNearbyDuplicate(int[] nums, int k) { HashMap<Integer, … Read more
Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers. Ensure that numbers within the set are sorted in ascending order. Example 1: Input: k = 3, n = 7 … Read more
After robbing those houses on that street, the thief has found himself a new place for his thievery so that he will not get too much attention. This time, all houses at this place are arranged in a circle. That means the first house is the neighbor of the last one. Meanwhile, the security system … Read more
Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element. For example, given [3,2,1,5,6,4] and k = 2, return 5. Note: You may assume k is always valid, 1 ≤ k ≤ array’s length. Java Solution 1 – Sorting … Read more
Given an array of integers, find if the array contains any duplicates. Your function should return true if any value appears at least twice in the array, and it should return false if every element is distinct. Java Solution public boolean containsDuplicate(int[] nums) { if(nums==null || nums.length==0) return false; HashSet<Integer> set = new HashSet<Integer>(); … Read more
Given a 2D binary matrix filled with 0’s and 1’s, find the largest rectangle containing all ones and return its area. Analysis This problem can be converted to the “Largest Rectangle in Histogram” problem. Java Solution public int maximalRectangle(char[][] matrix) { int m = matrix.length; int n = m == 0 ? 0 : matrix[0].length; … Read more
Given n non-negative integers representing the histogram’s bar height where the width of each bar is 1, find the area of largest rectangle in the histogram. Above is a histogram where width of each bar is 1, given height = [2,1,5,6,2,3]. The largest rectangle is shown in the shaded area, which has area = 10 … Read more
Follow up for “Unique Paths“: Now consider if some obstacles are added to the grids. How many unique paths would there be? An obstacle and empty space is marked as 1 and 0 respectively in the grid. For example, there is one obstacle in the middle of a 3×3 grid as illustrated below, [ [0,0,0], … Read more
Given an integer n, generate a square matrix filled with elements from 1 to n^2 in spiral order. For example, given n = 4, [ [1, 2, 3, 4], [12, 13, 14, 5], [11, 16, 15, 6], [10, 9, 8, 7] ] Java Solution 1 public int[][] generateMatrix(int n) { int total = n*n; int[][] … Read more
Given two numbers represented as strings, return multiplication of the numbers as a string. Analysis The key to solve this problem is multiplying each digit of the numbers at the corresponding positions and get the sum values at each position. That is how we do multiplication manually. Java Solution public String multiply(String num1, String num2) … Read more