225. Implement Stack using Queues LeetCode Solution

Last updated: February 15, 2025

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Problem Statement
Complexity Analysis
Implement Stack using Queues solution in C++
Implement Stack using Queues solution in Java
Implement Stack using Queues solution in Python
Additional Resources

225. Implement Stack using Queues LeetCode Solution image

Problem Statement of Implement Stack using Queues

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).
Implement the MyStack class:

void push(int x) Pushes element x to the top of the stack.
int pop() Removes the element on the top of the stack and returns it.
int top() Returns the element on the top of the stack.
boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue’s standard operations.

Example 1:

Input
[“MyStack”, “push”, “push”, “top”, “pop”, “empty”]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]

Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

1 <= x <= 9
At most 100 calls will be made to push, pop, top, and empty.
All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

Complexity Analysis

Time Complexity: O(1)
Space Complexity: O(n)

225. Implement Stack using Queues LeetCode Solution in C++

class MyStack {
 public:
  void push(int x) {
    q.push(x);
    for (int i = 0; i < q.size() - 1; ++i) {
      q.push(q.front());
      q.pop();
    }
  }

  int pop() {
    const int val = q.front();
    q.pop();
    return val;
  }

  int top() {
    return q.front();
  }

  bool empty() {
    return q.empty();
  }

 private:
  queue<int> q;
};
/* code provided by PROGIEZ */

225. Implement Stack using Queues LeetCode Solution in Java

class MyStack {
  public void push(int x) {
    q.offer(x);
    for (int i = 0; i < q.size() - 1; ++i)
      q.offer(q.poll());
  }

  public int pop() {
    return q.poll();
  }

  public int top() {
    return q.peek();
  }

  public boolean empty() {
    return q.isEmpty();
  }

  private Queue<Integer> q = new ArrayDeque<>();
}
// code provided by PROGIEZ

225. Implement Stack using Queues LeetCode Solution in Python

class MyStack:
  def __init__(self):
    self.q = collections.deque()

  def push(self, x: int) -> None:
    self.q.append(x)
    for _ in range(len(self.q) - 1):
      self.q.append(self.q.popleft())

  def pop(self) -> int:
    return self.q.popleft()

  def top(self) -> int:
    return self.q[0]

  def empty(self) -> bool:
    return not self.q
 # code by PROGIEZ

Additional Resources

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