```
int firstMissingPositive(int A[], int n) {
if(n==0)
return 1;
for (int i(0); i != n;)
{
if (A[i] != i + 1 && A[i]>0 && A[i] <= n&& A[A[i]-1] != A[i])
{
swap(A[i], A[A[i] - 1]);
}
else ++i;
}
for (int i(0); i != n; ++i)
{
if (A[i] != i + 1)
return i + 1;
}
return n+1;
}
```