Given a singly linked list, determine if it is a palindrome. Java Solution 1 – Creat a new reversed list We can create a new list in reversed order and then compare each node. The time and space are O(n). public boolean isPalindrome(ListNode head) { if(head == null) return true; ListNode p = head; … Read more
Implement the following operations of a queue using stacks. push(x) — Push element x to the back of queue. pop() — Removes the element from in front of queue. peek() — Get the front element. empty() — Return whether the queue is empty. Java Solution class MyQueue { Stack<Integer> temp = new Stack<Integer>(); Stack<Integer> … Read more
Given an integer, write a function to determine if it is a power of two. Analysis If a number is power of 2, it’s binary form should be 10…0. So if we right shift a bit of the number and then left shift a bit, the value should be the same when the number >= … Read more
Given an integer array of size n, find all elements that appear more than ⌊ n/3 ⌋ times. The algorithm should run in linear time and in O(1) space. Java Solution This problem is similar to Majority Element I. Time = O(n) and Space = O(1). public List<Integer> majorityElement(int[] nums) { List<Integer> result = new … Read more
Given a binary search tree, write a function kthSmallest to find the kth smallest element in it. (1 ≤ k ≤ BST’s total elements) Java Solution 1 – Inorder Traversal We can inorder traverse the tree and get the kth smallest element. Time is O(n). public int kthSmallest(TreeNode root, int k) { Stack<TreeNode> stack = … Read more
Given a sorted integer array without duplicates, return the summary of its ranges for consecutive numbers. For example, given [0,1,2,4,5,7], return [“0->2″,”4->5″,”7”]. Analysis When iterating over the array, two values need to be tracked: 1) the first value of a new range and 2) the previous value in the range. Java Solution public List<String> summaryRanges(int[] … Read more
Implement wildcard pattern matching with support for ‘?’ and ‘*’. Java Solution To understand this solution, you can use s=”aab” and p=”*ab”. public boolean isMatch(String s, String p) { int i = 0; int j = 0; int starIndex = -1; int iIndex = -1; while (i < s.length()) { if (j < p.length() … Read more
You are given a string, s, and a list of words, words, that are all of the same length. Find all starting indices of substring(s) in s that is a concatenation of each word in words exactly once and without any intervening characters. For example, given: s=”barfoothefoobarman” & words=[“foo”, “bar”], return [0,9]. Analysis This problem … Read more
Java Solution 1 – Recursive public TreeNode invertTree(TreeNode root) { helper(root); return root; } public void helper(TreeNode n){ if(n==null){ return; } TreeNode t = n.left; n.left = n.right; n.right = t; helper(n.left); helper(n.right); }public TreeNode invertTree(TreeNode root) { helper(root); return root; } public void helper(TreeNode n){ if(n==null){ return; } TreeNode t = … Read more
A message containing letters from A-Z is being encoded to numbers using the following mapping: ‘A’ -> 1 ‘B’ -> 2 … ‘Z’ -> 26 Given an encoded message containing digits, determine the total number of ways to decode it. Java Solution This problem can be solve by using dynamic programming. It is similar to … Read more
Implement the following operations of a stack using queues. push(x) — Push element x onto stack. pop() — Removes the element on top of the stack. top() — Get the top element. empty() — Return whether the stack is empty. Note: only standard queue operations are allowed, i.e., poll(), offer(), peek(), size() and isEmpty() in … Read more
Follow up for problem “Populating Next Right Pointers in Each Node“. What if the given tree could be any binary tree? Would your previous solution still work? Analysis Similar to Populating Next Right Pointers in Each Node, we have 4 pointers at 2 levels of the tree. Java Solution public void connect(TreeLinkNode root) { if(root … Read more
Implement a basic calculator to evaluate a simple expression string. The expression string may contain open ( and closing parentheses ), the plus + or minus sign -, non-negative integers and empty spaces.You may assume that the given expression is always valid. Some examples: “1 + 1” = 2, “(1)” = 1, “(1-(4-5))” = 2 … Read more
Find the total area covered by two rectilinear rectangles in a 2D plane. Each rectangle is defined by its bottom left corner and top right corner coordinates. Analysis This problem can be converted as a overlap internal problem. On the x-axis, there are (A,C) and (E,G); on the y-axis, there are (F,H) and (B,D). If … Read more
Analysis This problem is essentially a problem of processing 2*n edges. Each edge has a x-axis value and a height value. The key part is how to use the height heap to process each edge. Java Solution class Edge { int x; int height; boolean isStart; public Edge(int x, int height, boolean isStart) { … Read more