leetcode[41]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.

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