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) {
        int i,j;
        for(i=0;i<n;i++){
            int cur=A[i];
            if(cur==i+1||cur<=0||cur>n)continue;
            swap(A[i],A[cur-1]);
            if(A[i]!=A[cur-1])
                i--;
        }

        for(i=0;i<n;i++)
            if(A[i]!=i+1) 
                return i+1;
        return n+1;
    }
};