/**
 * 队列
 */
function Queue() {
    var items = [];
    this.enqueue = function(element){
        items.push(element);
    };
    this.dequeue = function(){
        return items.shift();
    };
    this.front = function(){
        return items[0];
    };
    this.end = function(){
		return items[items.length-1];
    }
    this.isEmpty = function(){
        return items.length == 0;
    };
    this.clear = function(){
        items = [];
    };
    this.size = function(){
        return items.length;
    };
    this.print = function(){
        console.log(items.toString());
    }; 
}