LeetCode – Palindrome Linked List (Java)

by

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

LeetCode – Implement Queue using Stacks (Java)

by

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

LeetCode – Range Addition (Java)

by

Assume you have an array of length n initialized with all 0’s and are given k update operations. Each operation is represented as a triplet: [startIndex, endIndex, inc] which increments each element of subarray A[startIndex … endIndex] (startIndex and endIndex inclusive) with inc. Return the modified array after all k operations were executed. For example, … Read more

LeetCode – Majority Element II (Java)

by

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

LeetCode – Kth Smallest Element in a BST (Java)

by

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

LeetCode – Summary Ranges (Java)

by

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

LeetCode – Wildcard Matching (Java)

by

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

LeetCode – Substring with Concatenation of All Words (Java)

by

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

LeetCode – Invert Binary Tree (Java)

by

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