fish.js

/**
given two non-empty arrays A and B consisting of N integers, returns the number of fish that will stay alive.

For example, given the arrays shown above, the function should return 2, as explained above.

Write an efficient algorithm for the following assumptions:

N is an integer within the range [1..100,000];
each element of array A is an integer within the range [0..1,000,000,000];
each element of array B is an integer that can have one of the following values: 0, 1;
the elements of A are all distinct.
*/

function solution(A, B) {
    var downStream = []
    var upStream = []
    var direction, UP = 0, DOWN = 1;
    
    for (var i = 0; i < A.length; i++) {
        direction = B[i]

        if (direction === DOWN) {
            // if fish goes down, line up those
            downStream.push(A[i]);
        } else  if (direction === UP) {
            // if fish goes up, meet all downstream fishes
            // if eats a downstream fish, meet next downstream fish
            // if eats all downstream fishes, survived!!
            // if eaten by bigger downstream fish, process next 
            const upFish = A[i];
            while (downStream.length > 0) {
                var downFish = downStream.slice(-1); // get the latest downFish
                if (downFish > upFish) { // downFish wins, process next
                    break
                } else {  // downFish eaten by upFish. Let's meet next downFish
                    downStream.pop();
                }
            }
            
            // upFish eats all downFishes, survived!!
            if (downStream.length === 0) { 
                upStream.push(upFish)
            }
        }
    }
    
    return downStream.length + upStream.length    
}