Implement Stack using Queues leetcode

mplement 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).

方法1: push() O(1), pop() O(n), peek() O(n) 用两个queue 来实现, 每次pop事后都把q1前边的都推入q2中, 最后一个出列, 然后q1, q2互换, top时候同理, 只是最后一个元素也入列q2.

class MyStack {
    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 tem = q1;
        q1 = q2;
        q2 = tem;
    }

    // Get the top element.
    public int top() {
        while (q1.size() > 1) {
            q2.offer(q1.poll());
        }
        int res = q1.peek();
        q2.offer(q1.poll());
        Queue tem = q1;
        q1 = q2;
        q2 = tem;
        return res;
    }

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

方法2: push() O(n), pop() O(1), peek() O(1) 用两个queue 来实现, 每次push时候都入列q2, 然后再把q1的元素都一一入列q2直到q1空为止, 然后q1 q2互换. 这样q1内元素的出列顺序和stack中的顺序相同 所以pop 和 top功能就是q1的 poll() 和peek()

class MyStack {
    Queue<Integer> q1 = new LinkedList<Integer>();
    Queue<Integer> q2 = new LinkedList<Integer>();
    public void push(int x) {
        q2.offer(x);
        while (!q1.isEmpty()) {
            q2.offer(q1.poll());
        }
        Queue tem = q1;
        q1 = q2;
        q2 = tem;
    }

    // Removes the element on top of the stack.
    public void pop() {
        q1.poll();
    }

    // Get the top element.
    public int top() {
        
        return q1.peek();
    }

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