Fish.cs

public static int solution(int[] A, int[] B)
{
    Stack<int> upstreamQueue = new Stack<int>(); // 0's
    Stack<int> downstreamQueue = new Stack<int>(); // 1's

    for (int i = 0; i < A.Length; i++)
    {
        if (B[i] == 0)  // is upstream              
        {
            if (downstreamQueue.Count > 0) // there is fish to eat
            {
                while (downstreamQueue.Count > 0 && downstreamQueue.Peek() < A[i])
                {
                    downstreamQueue.Pop();
                }

                if (downstreamQueue.Count == 0)
                {                            
                    upstreamQueue.Push(A[i]);
                }
            }
            else
            {
                upstreamQueue.Push(A[i]);
            }
        }
        else // is downstream
        {
            downstreamQueue.Push(A[i]);
        }
    }

    return upstreamQueue.Count + downstreamQueue.Count;
}