用两个栈实现队列

public class Solution {
  Stack<Integer> stack1 = new Stack<>();
  Stack<Integer> stack2 = new Stack<>();

  public void push(int node) {
    stack1.push(node);
  }

  public int pop() {
    if (stack1.empty() && stack2.empty()) {
      throw new RuntimeException("queue is empty");
    }
    if (stack2.empty()) {
      while (!stack1.empty()) {
        stack2.push(stack1.pop());
      }
    }
    return stack2.pop();
  }
}