[LeetCode] 225. Implement Stack using Queues

用队列实现栈。题干即是题意，这一题跟之前的232题一样，需要实现几个queue的函数。例子，

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

这一题只需要用到一个队列。因为是JS的关系所以这一题会好做很多，因为在JS里面，栈和队列都是用数组实现的。依然解释一下几个函数的实现方式。

push(x) - 将元素推进栈。时间均摊O(n)，因为涉及到会扩大内存的动作。
pop() -- 将栈顶元素弹出。时间O(1)
top() -- 等同于peek，看一下栈顶元素是什么。时间O(1)
empty() -- 判断栈是否为空。时间O(1)

JavaScript实现

/**
 * Initialize your data structure here.
 */
var MyStack = function () {
    this.stack = [];
};

/**
 * Push element x onto stack. 
 * @param {number} x
 * @return {void}
 */
MyStack.prototype.push = function (x) {
    this.stack[this.stack.length] = x;
};

/**
 * Removes the element on top of the stack and returns that element.
 * @return {number}
 */
MyStack.prototype.pop = function () {
    let res = this.stack[this.stack.length - 1];
    this.stack.length = this.stack.length - 1;
    return res;
};

/**
 * Get the top element.
 * @return {number}
 */
MyStack.prototype.top = function () {
    return this.stack[this.stack.length - 1];
};

/**
 * Returns whether the stack is empty.
 * @return {boolean}
 */
MyStack.prototype.empty = function () {
    return this.stack.length === 0;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * var obj = new MyStack()
 * obj.push(x)
 * var param_2 = obj.pop()
 * var param_3 = obj.top()
 * var param_4 = obj.empty()
 */

Java实现

class MyStack {
    Queue<Integer> queue;
    /** Initialize your data structure here. */
    public MyStack() {
        queue = new LinkedList<>();
    }
    
    /** Push element x onto stack. */
    public void push(int x) {
        queue.add(x);
        for (int i = 0; i < queue.size() - 1; i++) {
            queue.add(queue.poll());
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    public int pop() {
        return queue.poll();
    }
    
    /** Get the top element. */
    public int top() {
        return queue.peek();
    }
    
    /** Returns whether the stack is empty. */
    public boolean empty() {
        return queue.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();
 */

LeetCode 题目总结