## Description

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

将k个有序链表合并为一个链表

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

将k个有序链表合并为一个链表

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

`[ "((()))", "(()())", "(())()", "()(())", "()()()" ]`

给出n组括号，生成所有正确的匹配组合

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.

将两个有序链表合二为一

Given a string containing just the characters

`'('`

,`')'`

,`'{'`

,`'}'`

,`'['`

and`']'`

, determine if the input string is valid.

The brackets must close in the correct order,`"()"`

and`"()[]{}"`

are all valid but`"(]"`

and`"([)]"`

are not.

给出包含三种括号的字符串，验证括号是否左右匹配

Remove Nth Node From End of List

Given a linked list, remove the nth node from the end of list and return its head.

`For example, Given linked list: 1->2->3->4->5, and n = 2. After removing the second node from the end, the linked list becomes 1->2->3->5.`

Note: Given n will always be valid. Try to do this in one pass.

从链表中删除倒数第n个结点（n必定有效）

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: The solution set must not contain duplicate quadruplets.`For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0. A solution set is: [ [-1, 0, 0, 1], [-2, -1, 1, 2], [-2, 0, 0, 2] ]`

Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

`Input: Digit string "23" Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].`

Programer

ChengDu