LeetCode 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.

      O(n) 时间，并且不能开辟数组。我们通过交换数组中的值，尽量使得A[i]=i+1，这样，当交换完后，再扫描一下数组，如果A[i]!=i+1，则i+1为第一个missing的正整数。

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