题解 | #用两个栈实现队列#

push O(1)，pop O(n)...

import java.util.Stack;

public class Solution {
    Stack<Integer> stack1 = new Stack<Integer>();
    Stack<Integer> stack2 = new Stack<Integer>();
    
     public void push(int node) {
        stack1.push(node);
    }

    public int pop() {
        if (stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
        }

        if (!stack2.isEmpty()) {
            return stack2.pop();
        }
        return -1;
    }
}