First Missing Positive

//from anniekim

Solution: Although we can only use constant space, we can still exchange elements within input A!

Swap elements in A and try to make all the elements in A satisfy: A[i] == i + 1.

Pick out the first one that does not satisfy A[i] == i + 1.

 

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