232. Implement Queue using Stacks

/**
 * Initialize your data structure here.
 */
var MyQueue = function() {
    this.stack1 = [];
    this.stack2 = [];
};

/**
 * Push element x to the back of queue. 
 * @param {number} x
 * @return {void}
 */
MyQueue.prototype.push = function(x) {
    this.stack1.push(x);
};

/**
 * Removes the element from in front of queue and returns that element.
 * @return {number}
 */
MyQueue.prototype.pop = function() {
    this.checkStack();
    return this.stack2.pop();
};

/**
 * Get the front element.
 * @return {number}
 */
MyQueue.prototype.peek = function() {
     this.checkStack();
     return this.stack2[this.stack2.length - 1];
};
MyQueue.prototype.checkStack = function() {
    if(!this.stack2.length) {
        while(this.stack1.length) {
            this.stack2.push(this.stack1.pop());
        }
    }
}
/**
 * Returns whether the queue is empty.
 * @return {boolean}
 */
MyQueue.prototype.empty = function() {
    return (this.stack2.length == 0 && this.stack1.length == 0);
};

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