LeetCode：Implement Stack using Queues

问题描述：

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 emptyoperations 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)

思路：

很基础的一道题目，利用两个队列实现一个栈。

java代码：

class MyStack {
    // Push element x onto stack.
    Queue<Integer> q1 = new LinkedList<>(); 
    Queue<Integer> q2 = new LinkedList<>(); 
    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();
    }
}