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;

}
};