function fish(a: number[], b: number[]): number {
  const size = a.length;
  let dead = 0;
  const downstream: number[] = [];

  for (let i = 0; i < size; i++) {
    if (b[i] === 1) {
      downstream.push(a[i]);
    } else if (downstream.length > 0) {
      while (downstream.length > 0) {
        dead++;
        if (a[i] > downstream[downstream.length - 1]) {
          downstream.pop();
        } else {
          break;
        }
      }
    }
  }

  return size - dead;
}