This is a problem asked by Google. Given a string, find the longest substring that contains only two unique characters. For example, given “abcbbbbcccbdddadacb”, the longest substring that contains 2 unique character is “bcbbbbcccb”. 1. Longest Substring Which Contains 2 Unique Characters In this solution, a hashmap is used to track the unique elements in … Read more
Determine whether an integer is a palindrome. Do this without extra space. Thoughts Problems related with numbers are frequently solved by / and %. No need of extra space is required. This problem is similar with the Reverse Integer problem. Note: no extra space here means do not convert the integer to string, since string … Read more
Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for “abcabcbb” is “abc”, which the length is 3. For “bbbbb” the longest substring is “b”, with the length of 1. Analysis The basic idea to solve this problem is using an extra data … Read more
Given a collection of numbers that might contain duplicates, return all possible unique permutations. For example, [1,1,2] have the following unique permutations: [1,1,2], [1,2,1], and [2,1,1]. Java Solution 1 Based on Permutation, we can add a set to track if an element is duplicate and no need to swap. public List<List<Integer>> permuteUnique(int[] nums) { List<List<Integer>> … Read more
Given a collection of numbers, return all possible permutations. For example, [1,2,3] have the following permutations: [1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1]. Java Solution 1 – Iteration We can get all permutations by the following steps: [1] [2, 1] [1, 2] [3, 2, 1] [2, 3, 1] [2, 1, 3] [3, 1, 2] [1, … Read more
Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target. Note: Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ … Read more
Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution. For example, given array S = {-1 2 1 -4}, and target = 1. … Read more
Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity. Analysis The simplest solution is using PriorityQueue. The elements of the priority queue are ordered according to their natural ordering, or by a comparator provided at the construction time (in this case). Java Solution public ListNode mergeKLists(ListNode[] lists) … Read more
Given a linked list and a value x, partition it such that all nodes less than x come before nodes greater than or equal to x. You should preserve the original relative order of the nodes in each of the two partitions. For example, given 1->4->3->2->5->2 and x = 3, return 1->2->2->4->3->5. Java Solution public … Read more
The set [1,2,3,…,n] contains a total of n! unique permutations. By listing and labeling all of the permutations in order, We get the following sequence (ie, for n = 3): “123” “132” “213” “231” “312” “321” Given n and k, return the kth permutation sequence. (Note: Given n will be between 1 and 9 inclusive.) … Read more
Given a binary tree, determine if it is height-balanced. For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1. Analysis This is a typical tree problem that can be solve by using recursion. Java Solution … Read more
Given a binary tree, find the maximum path sum. The path may start and end at any node in the tree. For example, given the below binary tree 1 / \ 2 3 the result is 6. Analysis 1) Recursively solve this problem 2) Get largest left sum and right sum 2) Compare to the … Read more
Given a binary tree, find its minimum depth. The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node. Thoughts LinkedList is a queue in Java. The add() and remove() methods are used to manipulate the queue. Java Solution /** * Definition for binary … Read more
Problem: Get maximum binary Gap. For example, 9’s binary form is 1001, the gap is 2. Java Solution 1 An integer x & 1 will get the last digit of the integer. public static int getGap(int N) { int max = 0; int count = -1; int r = 0; while (N > 0) … Read more
Find the contiguous subarray within an array (containing at least one number) which has the largest sum. For example, given the array [−2,1,−3,4,−1,2,1,−5,4], the contiguous subarray [4,−1,2,1] has the largest sum = 6. Java Solution – DP The easiest way to formulate the solution of this problem is using DP. Let f(n) be the maximum … Read more