https://www.acmicpc.net/problem/10773

제로.java

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int K = sc.nextInt();
        Stack<Integer> stack = new Stack<>();
        for(int i=0; i<K; i++){
            int input = sc.nextInt();
            if(input==0) stack.pop();
            else stack.push(input);
        }
        sc.close();
        int sum = 0;
        for (Integer i : stack)
            sum+=i; 

        System.out.print(sum);
    }
}