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) {
// Start typing your C/C++ solution below
// DO NOT write int main() function
int outOfRange = n + 2;
for(int i = 0; i< n; i++)
if(A[i]<=0)
A[i] = outOfRange ;
int absi;
for(int i = 0; i< n; i++)
{
absi = abs(A[i]);
if(absi <= n)
{
A[absi-1] = -abs(A[absi-1]) ;
}
}

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