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 && 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;    }};