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(n==0) return 1;
        
        for(int i=0;i<n;i++)
            if(A[i]!=i+1 && A[i]-1>=0 && A[i]-1<n && A[A[i]-1]!=A[i])
            {
                int tmp=A[A[i]-1];
                A[A[i]-1]=A[i];
                A[i]=tmp;
                i--;
            }
            
        for(int i=0;i<n;i++)
            if(A[i]!=i+1) return i+1;
        return n+1;
        
    }
};