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 {
    private Queue<Integer> q1 = new LinkedList<>();
    private Queue<Integer> q2 = new LinkedList<>();
    private int top;

    /** Initialize your data structure here. */
    // public MyStack() {
    // }

    /** Push element x onto stack. */
    public void push(int x) {
        q1.offer(x);
        top = x;
    }

    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        while (q1.size() > 1) {
        top = q1.remove();
        q2.add(top);
        }
        int e=q1.remove();
        Queue<Integer> temp = q2;
        q2 = q1;
        q1 = temp;
        return e;
    }

    /** Get the top element. */
    public int top() {
        return top;
    }

    /** Returns whether the stack is empty. */
    public boolean empty() {
        return q1.isEmpty();
    }
}

/**
 * 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();
 */