Given a non-negative number represented as an array of digits, plus one to the number. The digits are stored such that the most significant digit is at the head of the list. Java Solution To solve this problem, we can use a flag to mark if the current digit needs to be changed. public int[] … Read more
Given the following perfect binary tree, 1 / \ 2 3 / \ / \ 4 5 6 7 After calling your function, the tree should look like: 1 -> NULL / \ 2 -> 3 -> NULL / \ / \ 4->5->6->7 -> NULL Java Solution 1 – Simple public void connect(TreeLinkNode root) { … Read more
Given a string s consists of upper/lower-case alphabets and empty space characters ‘ ‘, return the length of last word in the string. If the last word does not exist, return 0. Java Solution We can scan the string from the end. When there is a letter, start counting the number of letters. This should … Read more
Given two binary strings, return their sum (also a binary string). For example, a = “11”, b = “1”, the return is “100”. Java Solution Very simple, nothing special. Note how to convert a character to an int. public String addBinary(String a, String b) { if(a==null || a.length()==0) return b; if(b==null || b.length()==0) return a; … Read more
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
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