[LeetCode][JavaScript]Implement Stack using Queues

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).

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

https://leetcode.com/problems/implement-stack-using-queues/

 

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用队列模拟栈。

只能用队列的入队，出队，长度和是否为空四个功能。

也就是js数组的push(),shift(),length,length===0。

 

两个队列倒来倒去。

入栈操作就是找到不为空的队列，入队。

出栈和查看栈顶元素的操作是一样的，把不为空的那个队列，除了最后一个元素的其他所有元素都倒到另一个里去。

栈是否为空就是两个队列是否都为空。

 

/**
 * @constructor
 */
var Stack = function() {
    this.queueA = [];
    this.queueB = [];
};

/**
 * @param {number} x
 * @returns {void}
 */
Stack.prototype.push = function(x) {
    if(this.queueA.length !== 0){
        this.queueA.push(x);
    }else{
        this.queueB.push(x);
    }
};

Stack.prototype.moveAToB = function() {
    if(this.queueB.length !== 0){
        var tmp = this.queueA;
        this.queueA = this.queueB;
        this.queueB = tmp;
    }
    var len = this.queueA.length;
    for(var i = 0; i < len - 1; i++){
        var queuePeek = this.queueA.shift();
        this.queueB.push(queuePeek);
    }
};

/**
 * @returns {void}
 */
Stack.prototype.pop = function() {
    this.moveAToB();
    this.queueA.shift();
};

/**
 * @returns {number}
 */
Stack.prototype.top = function() {
    this.moveAToB();
    var stackTop = this.queueA.shift();
    this.queueB.push(stackTop);
    return stackTop;
};

/**
 * @returns {boolean}
 */
Stack.prototype.empty = function() {
    if(this.queueA.length === 0 && this.queueB.length === 0){
        return true;
    }
    return false;
};