队列的enqueue方法和dequeue方法添加和移除元素 ，原理是利用array的push()和shift
    ()实现

    完整的Queue类
    function Queue() {
    var items = [];

    1.     this.enqueue = function(element){<br />            items.push(element);

    };
    this.dequeue = function(){
    return items.shift();

    };
    this.front = function(){
    return items[0];

    };
    this.isEmpty = function(){
    return items.length == 0;

    };
    this.clear = function(){
    items = [];

    };
    this.size = function(){
    return items.length;

    };
    this.print = function(){
    console.log(items.toString());
    };
    }