Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water. Example 1: {代码...} Answer: 1 Example 2...

Count consecutive digits and say it. For example, return 132341 if input is 1112224. There are three 1s, three 2s and one 4.

Find the nth to last element of a singly linked list. The minimum number of nodes in list is n.

Given a sorted array, remove the duplicates in place such that each element appear only once and return the new length. Do not allocate extra space for another array, you must do this in place with constant memory. For example, Given input array nums = [1,1,2], Your function should return length ...

Given a binary tree, find its maximum depth. The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Design a stack that supports push, pop, top, and retrieving theminimum element in constant time.push(x) -- Push element x onto stack. pop() -- Removes the element ontop of the stack. top() -- Get the top element. getMin() -- Retrievethe minimum element in the stack.

Given a m x n matrix, if an element is 0, set its entire row and column to 0. Do it in place. click to show follow up. Follow up: Did you use extra space? A straight forward solution using O(mn) space is probably a bad idea. A simple improvement uses O(m + n) space, but still not the best solutio...

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T. The same repeated number may be chosen from C unlimited number of times. Note: All numbers (including target) will be positive integers. Elements in a combination...

Given two integers n and k, return all possible ombinations of k numbers out of 1 ... n. For example, If n = 4 and k = 2, a solution is: {代码...}

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set. get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1. set(key, value) - Set or insert the value if the k...

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 g...

Given two words word1 and word2, find the minimum number of steps required to convert word1 to word2. (each operation is counted as 1 step.) You have the following 3 operations permitted on a word: a) Insert a character b) Delete a character c) Replace a character

Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors. OJ's undirected graph serialization: Nodes are labeled uniquely. We use # as a separator for each node, and , as a separator for node label and each neighbor of the node. As an example, consider the se...

先算出两个链表各自的长度，然后从较长的链表先遍历，遍历到较长链表剩余长度和较短链表一样时，用两个指针同时遍历两个链表。这样如果链表有交点的话，两个指针已经一定会相遇。

Given an array of strings, group anagrams together. For example, given: ["eat", "tea", "tan", "ate", "nat", "bat"], Return: {代码...}

Given an array of size n, find the majority element. The majority element is the element that appears more than ⌊ n/2 ⌋ times. You may assume that the array is non-empty and the majority element always exist in the array.

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.For example, Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 u...

3Sum其实可以转化成一个2Sum的题，我们先从数组中选一个数，并将目标数减去这个数，得到一个新目标数。然后再在剩下的数中找一对和是这个新目标数的数，其实就转化为2Sum了。为了避免得到重复结果，我们不仅要跳过重复元素，而且要保证2Sum找的范围要是在我们最先选定的那个数之后的。

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. Notes: You must use only standard operations of a stack --...

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Given a linked list, determine if it has a cycle in it.Follow up: Can you solve it without using extra space?

插排的思路是保证遍历到每个元素时，该元素前面的序列都是有序的。基于这个前提，我们将遍历到的新元素和它前面的序列相比对，然后将这个元素插入到适当的位置，使得这个包含新元素的序列也是有序的。虽然外层遍历只要O(N)时间，但是因为找到新元素的适当位置后要将之后的所有元素后移，所以最差时间是O(N^2）。

What is singleton? What's its cons and pros? How to implement it?Definition: Singleton pattern is a design pattern that ensure that only one instance of a class is created and Provide a global access point to the object.

首先我们要熟悉罗马数的表达方式。M是1000，D是500，C是100，L是50，X是10，V是5，I是1。验证字符串是否是罗马数，我们先看一下有效的罗马数是什么样的，假设该数字小于5000，从千位到个位依次拆解。千位的表达方式 M{0,4}

What's the difference between thread and process?A process is an instance of a computer program that is being executed. It contains the program code and its current activity. A process may be made up of multiple threads.

Implement a trie with insert, search, and startsWith methods. Note: You may assume that all inputs are consist of lowercase letters a-z.

Median is the middle value in an ordered integer list. If the size of the list is even, there is no middle value. So the median is the mean of the two middle value.Examples: [2,3,4] , the median is 3 [2,3], the median is (2 + 3) / 2 = 2.5 Design a data structure that supports the following two op...

Given a 2D binary matrix filled with 0's and 1's, find the largest square containing all 1's and return its area. For example, given the following matrix: {代码...} Return 4.

Given an array of integers, find if the array contains any duplicates. Your function should return true if any value appears at least twice in the array, and it should return false if every element is distinct.

Given two strings s and t, determine if they are isomorphic.Two strings are isomorphic if the characters in s can be replaced to get t. All occurrences of a character must be replaced with another character while preserving the order of characters. No two characters may map to the same character ...