Given n nodes labeled from 0 to n – 1 and a list of undirected edges (each edge is a pair of nodes), check if these edges form a valid tree. Analysis This problem can be converted to finding the cycle from a graph. It can be solved by using DFS (Recursion) or BFS (Queue). … Read more
Given a binary tree, find its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node. Java Solution public int maxDepth(TreeNode root) { if(root==null) return 0; int leftDepth = maxDepth(root.left); int rightDepth = maxDepth(root.right); int bigger = Math.max(leftDepth, rightDepth); … Read more
Given a binary tree, return all root-to-leaf paths. 1. Naive DFS Solution A typical depth-first search problem. public List<String> binaryTreePaths(TreeNode root) { ArrayList<String> finalResult = new ArrayList<String>(); if(root==null) return finalResult; ArrayList<String> curr = new ArrayList<String>(); ArrayList<ArrayList<String>> results = new ArrayList<ArrayList<String>>(); dfs(root, results, curr); for(ArrayList<String> al : results){ StringBuilder sb = new … Read more
Given a non-empty binary search tree and a target value, find the value in the BST that is closest to the target. Java Solution 1 – Recursion Recursively traverse down the root. When target is less than root, go left; when target is greater than root, go right. public class Solution { int goal; double … Read more
Given a pattern and a string str, find if str follows the same pattern. Here follow means a full match, such that there is a bijection between a letter in pattern and a non-empty word in str. Java Solution public boolean wordPattern(String pattern, String str) { String[] arr = str.split(" "); //prevent out of … Read more
Given a nested list of integers, implement an iterator to flatten it. Each element is either an integer, or a list — whose elements may also be integers or other lists. For example, given the list [[1,1],2,[1,1]], by calling next repeatedly until hasNext returns false, the order of elements returned by next should be: [1,1,2,1,1]. … Read more
Given an array of integers, write a method to return the k most frequent elements. Java Solution 1 – Heap Time complexity is O(n*log(k)). Note that heap is often used to reduce time complexity from n*log(n) (see solution 3) to n*log(k). public List<Integer> topKFrequent(int[] nums, int k) { //count the frequency for each element HashMap<Integer, … Read more
Given a string, we can “shift” each of its letter to its successive letter, for example: “abc” -> “bcd”. We can keep “shifting” which forms the sequence: “abc” -> “bcd” -> … -> “xyz”. Given a list of strings which contains only lowercase alphabets, group all strings that belong to the same shifting sequence, return: … Read more
Given two strings s and t, write a function to determine if t is an anagram of s. Java Solution 1 Assuming the string contains only lowercase alphabets, here is a simple solution. public boolean isAnagram(String s, String t) { if(s==null || t==null) return false; if(s.length()!=t.length()) return false; int[] arr = new int[26]; … Read more
Given an array nums, write a function to move all 0’s to the end of it while maintaining the relative order of the non-zero elements. For example, given nums = [0, 1, 0, 3, 12], after calling your function, nums should be [1, 3, 12, 0, 0]. Java Solution 2 We can use the similar … Read more
Given an integer matrix, get the length of the longest increasing path. https://leetcode.com/problems/longest-increasing-path-in-a-matrix/ Java Solution 1 – Naive DFS public int longestIncreasingPath(int[][] matrix) { int[] max = new int[1]; for (int i = 0; i < matrix.length; i++) { for (int j = 0; j < matrix[0].length; j++) { dfs(matrix, i, j, max, 1); } … Read more
You are playing the following Flip Game with your friend: Given a string that contains only these two characters: + and -, you and your friend take turns to flip two consecutive “++” into “–“. The game ends when a person can no longer make a move and therefore the other person will be the … Read more
Given a stream of integers and a window size, calculate the moving average of all integers in the sliding window. Java Solution This problem is solved by using a queue. class MovingAverage { double sum; int size; LinkedList<Integer> list; /** Initialize your data structure here. */ public MovingAverage(int size) { this.list = new LinkedList<>(); … Read more
You are playing the following Flip Game with your friend: Given a string that contains only these two characters: + and -, you and your friend take turns to flip two consecutive “++” into “–“. The game ends when a person can no longer make a move and therefore the other person will be the … Read more
You are playing the following Nim Game with your friend: There is a heap of stones on the table, each time one of you take turns to remove 1 to 3 stones. The one who removes the last stone will be the winner. You will take the first turn to remove the stones. Analysis Java … Read more