This task is a training on how to use stacks https://app.codility.com/programmers/lessons/7-stacks_and_queues/fish/
The two main important things to notice so you can complete the task are:
- A fish going upstream will NEVER meet a downstream fish that comes next
- A upstream fish can ether eat all the downstream fish or die
This is a possible solution using one stack:
def solution(A,B): alive = len(A) downstream = [] for size,direction in zip(A,B): if direction == 1: downstream.append(size) else: while downstream: if size < downstream[-1]: # upstream fish is eaten alive = alive - 1 break else: # upstream fish eats the other one downstream.pop() alive = alive - 1 return alive 
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