225. Implement Stack using Queues

by hooni posted May 05, 2020


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Implement the following operations of a stack using queues.

  • push(x) -- Push element x onto stack.
  • pop() -- Removes the element on top of the stack.
  • top() -- Get the top element.
  • empty() -- Return whether the stack is empty.


MyStack stack = new MyStack();

stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false


  • You must use only standard operations of a queue -- which means only push to backpeek/pop from frontsize, and is empty operations are valid.
  • Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
  • You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

class MyStack {
    List<Integer> list;

    /** Initialize your data structure here. */
    public MyStack() {
        list = new ArrayList<>();
    /** Push element x onto stack. */
    public void push(int x) {
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return list.remove(list.size()-1);
    /** Get the top element. */
    public int top() {
        return list.get(list.size()-1);
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return list.size() == 0;

 * Your MyStack object will be instantiated and called as such:
 * MyStack obj = new MyStack();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.top();
 * boolean param_4 = obj.empty();

[문제] https://leetcode.com/problems/implement-stack-using-queues/