This is a classic interview question. Another similar problem is “merge k sorted lists“. This problem can be solved by using a heap. The time complexity is O(nlog(k)), where n is the total number of elements and k is the number of arrays. It takes O(log(k)) to insert an element to the heap and it … Read more
The string “PAYPALISHIRING” is written in a zigzag pattern on a given number of rows like this: (you may want to display this pattern in a fixed font for better legibility) P A H N A P L S I I G Y I R And then read line by line: “PAHNAPLSIIGYIR” Write the a … Read more
Write a program to solve a Sudoku puzzle by filling the empty cells. Java Solution public void solveSudoku(char[][] board) { helper(board); } private boolean helper(char[][] board){ for(int i=0; i<9; i++){ for(int j=0; j<9; j++){ if(board[i][j]!=’.’){ continue; } for(char k=’1′; k<=’9′; k++){ board[i][j]=k; if(isValid(board, i, j) && helper(board)){ return true; } board[i][j]=’.’; } … Read more
Determine if a Sudoku is valid. The Sudoku board could be partially filled, where empty cells are filled with the character ‘.’. Java Solution public boolean isValidSudoku(char[][] board) { if (board == null || board.length != 9 || board[0].length != 9) return false; // check each column for (int i = 0; i < 9; … Read more
Given a linked list, remove the nth node from the end of list and return its head. For example, given linked list 1->2->3->4->5 and n = 2, the result is 1->2->3->5. Java Solution 1 – Naive Two Passes Calculate the length first, and then remove the nth from the beginning. public ListNode removeNthFromEnd(ListNode head, int … Read more
Given a binary tree and a sum, find all root-to-leaf paths where each path’s sum equals the given sum. For example, given the below binary tree and sum = 22, 5 / \ 4 8 / / \ 11 13 4 / \ / \ 7 2 5 1 the method returns the following: [ … Read more
Design a data structure that supports the following two operations: void addWord(word) bool search(word) search(word) can search a literal word or a regular expression string containing only letters a-z or .. A . means it can represent any one letter. Java Solution 1 This problem is similar with Implement Trie. The solution 1 below uses … Read more
The gray code is a binary numeral system where two successive values differ in only one bit. Given a non-negative integer n representing the total number of bits in the code, print the sequence of gray code. A gray code sequence must begin with 0. For example, given n = 2, return [0,1,3,2]. Its gray … Read more
This list summarizes the top 10 mistakes that Java developers frequently make.
Given a nested list of integers, return the sum of all integers in the list weighted by their depth. Each element is either an integer, or a list — whose elements may also be integers or other lists. Example 1: Given the list [[1,1],2,[1,1]], return 10. (four 1’s at depth 2, one 2 at depth … Read more
Remove the minimum number of invalid parentheses in order to make the input string valid. Return all possible results. Note: The input string may contain letters other than the parentheses ( and ). Examples: “()())()” -> [“()()()”, “(())()”] “(a)())()” -> [“(a)()()”, “(a())()”] “)(” -> [“”] Java Solution This problem can be solve by using DFS. … Read more
Given n nodes labeled from 0 to n – 1 and a list of undirected edges (each edge is a pair of nodes), write a function to find the number of connected components in an undirected graph. For example: 0 3 | | 1 — 2 4 Given n = 5 and edges = [[0, … Read more
Given a linked list, reverse the nodes of a linked list k at a time and return its modified list. If the number of nodes is not a multiple of k then left-out nodes in the end should remain as it is. You may not alter the values in the nodes, only nodes itself may … Read more
Write a program to find the nth super ugly number. Super ugly numbers are positive numbers whose all prime factors are in the given prime list primes of size k. For example, [1, 2, 4, 7, 8, 13, 14, 16, 19, 26, 28, 32] is the sequence of the first 12 super ugly numbers given … Read more
There are a row of n houses, each house can be painted with one of the k colors. The cost of painting each house with a certain color is different. You have to paint all the houses such that no two adjacent houses have the same color. The cost of painting each house with a … Read more