[leetcode]First Missing Positive

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;
    }
};