Leetcode 225. Implement Stack using Queues

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.

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, 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 {    // Push element x onto stack.    Queue<Integer> q1 = new LinkedList<Integer>();    Queue<Integer> q2 = new LinkedList<Integer>();    public void push(int x) {      q1.offer(x);    }    // Removes the element on top of the stack.    public void pop() {      while(q1.size() > 1) q2.offer(q1.poll());      q1.poll();      Queue<Integer> q = q1;      q1 = q2;      q2 = q;    }    // Get the top element.    public int top() {      while(q1.size() > 1) q2.offer(q1.poll());      int x = q1.poll();      q2.offer(x);      Queue<Integer> q = q1;      q1 = q2;      q2 = q;      return x;    }    // Return whether the stack is empty.    public boolean empty() {      return q1.isEmpty();    }}