LeetCode First Missing Positive

题目要求：

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.

思路：把出现的数值放到与下标一致的位置，再判断什么位置最先出现不连续的数值。

代码：

class Solution {public:  int firstMissingPositive(int A[], int n)   {    if(A == NULL || n == 0)      return 1;    for(int i = 0; i < n; ++i)    {      if(A[i] < n && A[i] > 0)      {        if(A[i] != A[A[i] - 1])        {          swap(A[i], A[A[i] - 1]);          --i;        }      }    }    for(size_t i = 0; i < n; ++i)    {      if((A[i] - 1) != i)        return (i + 1);    }    return (n + 1);  }};