Month: April 2014

LeetCode – Binary Tree Level Order Traversal (Java)

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Given a binary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level). For example: Given binary tree {3,9,20,#,#,15,7}, 3 / \ 9 20 / \ 15 7 return its level order traversal as [[3], [9,20], [15,7]] Java Solution 1 It is obvious that this problem can … Read more

LeetCode – Letter Combinations of a Phone Number (Java)

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Given a digit string, return all possible letter combinations that the number could represent. (Check out your cellphone to see the mappings) Input:Digit string “23”, Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”]. Java Solution 1 – DFS This problem can be solves by a typical DFS algorithm. DFS problems are very similar … Read more

LeetCode – Pascal’s Triangle II (Java)

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Given an index k, return the kth row of the Pascal’s triangle. For example, when k = 3, the row is [1,3,3,1]. Analysis This problem is related to Pascal’s Triangle which gets all rows of Pascal’s triangle. In this problem, only one row is required to return. Java Solution public List<Integer> getRow(int rowIndex) { ArrayList<Integer> … Read more

LeetCode – Palindrome Partitioning II (Java)

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Given a string s, partition s such that every substring of the partition is a palindrome. Return the minimum cuts needed for a palindrome partitioning of s. For example, given s = “aab”, return 1 since the palindrome partitioning [“aa”,”b”] could be produced using 1 cut. Analysis This problem is similar to Palindrome Partitioning. It … Read more

LeetCode – Search a 2D Matrix II (Java)

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Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties: Integers in each row are sorted in ascending from left to right. Integers in each column are sorted in ascending from top to bottom. For example, consider the following matrix: [ [1, 4, 7, … Read more

How to Check if an Array Contains a Value in Java Efficiently?

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How to check if an array (unsorted) contains a certain value? This is a very useful and frequently used operation in Java. It is also a top voted question on Stack Overflow. As shown in top voted answers, this can be done in several different ways, but the time complexity could be very different. In the following I will show the time cost of each method.

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LeetCode – Longest Increasing Subsequence (Java)

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Given an unsorted array of integers, find the length of longest increasing subsequence. For example, given [10, 9, 2, 5, 3, 7, 101, 18], the longest increasing subsequence is [2, 3, 7, 101]. Therefore the length is 4. Java Solution 1 – Naive Let max[i] represent the length of the longest increasing subsequence so far. … Read more