Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of …

Given an array nums of n integers and an integer target, find three integers in nums such that the sum is closest to target. Return the sum of the three integers. You may assume that each input wou…

Given a string containing digits from 2-9 inclusive, return all possible letter combinations that the number could represent.

给定一颗根结点为 root 的二叉树，书中的每个结点都有一个从 0 到 25 的值，分别代表字母 ‘a’ 到 ‘z’：值 0 代表 ‘a’，值 1 代表 ‘b’，依此类推。 找出按字典序最小的字符串，该字符串从这棵树的一个叶结点开始，…

Given two binary trees, write a function to check if they are the same or not.

Given two sorted integer arrays nums1 and nums2, merge nums2 into nums1 as one sorted array.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

You are climbing a stair case. It takes n steps to reach to the top.Each time you can either climb 1 or 2 steps. In how many distinct ways can you climb to the top?