### LeetCode 23：Merge k Sorted Lists

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

Example:

```Input:
[
1->4->5,
1->3->4,
2->6
]

### LeetCode 22：Generate Parentheses

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:

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

### LeetCode 21：Merge Two Sorted Lists

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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->[……]

### LeetCode 20：Valid Parentheses

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[……]

### LeetCode 19：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 t[......]Read more```

### LeetCode 17：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 "2[......]Read more`

### LeetCode 16：3Sum Closest

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[……]