[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.

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