First Missing Positive Given an unsorted integer array, find the first missing positive integer. For example, Given [1,2,0] return 3, and [3,4,-1,1] return 2. Your algorithm should run in O(n) time and uses constant space. 关键是只允许常数空间。这里要突破思维定势，要改变给定数组的结构。 对于A[i]，如果A[i] > 0 && A[i] <= n，则将它与A[i] – 1 位置的元素互换位置。 遍历一遍后，再从左到右找到第一个A[i] != i + […]

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. 简单题，归并排序即可。 /** * Definition for singly-linked list. * struct ListNode { * int val; * ListNode *next; * […]