N voracious fish are moving along a river. Calculate how many fish are alive.

Fish.java

import java.util.Stack;

public class Fish {
	public int solution(int[] A, int[] B) {
		if(A == null || B == null || A.length != B.length) {
			return -1;
		}
		
		Stack<Integer> downstream = new Stack<>(); 
		int alive = 0;
		
		for(int i = 0; i < A.length; i++) {
			if(B[i] == 0) {
				while(!downstream.isEmpty()) {
					if(downstream.peek() < A[i]) {
						downstream.pop();
					} else {
						break;
					}
				}
				
				if(downstream.isEmpty()) {
					alive++;
				}
			} else {
				downstream.push(A[i]);
			}
		}
		
		alive += downstream.size();
		
		return alive;
	}
}