腾讯：用两个栈实现一个队列

var CQueue = function() {
    this.s = [];
    this.t = [];
};
CQueue.prototype.appendTail = function(value) {
    this.s.push(value);
};
CQueue.prototype.deleteHead = function(){
    if (this.t.length) {
        return this.t.pop();
    }
    while(this.s.length){
        this.t.push(this.s.pop());
    }
    return this.t.pop();
}