The key here is to use swapping to keep constant space and also make use of the length of the array, which means there can be at most n positive integers. So each time we encounter an valid integer, find its correct position and swap. Otherwise we continue.

```
public class Solution {
public int firstMissingPositive(int[] A) {
int i = 0;
while(i < A.length){
if(A[i] == i+1 || A[i] <= 0 || A[i] > A.length) i++;
else if(A[A[i]-1] != A[i]) swap(A, i, A[i]-1);
else i++;
}
i = 0;
while(i < A.length && A[i] == i+1) i++;
return i+1;
}
private void swap(int[] A, int i, int j){
int temp = A[i];
A[i] = A[j];
A[j] = temp;
}
}
```