隊列是一種線性表，按照先進先出的原則進行
單向隊列：只能從頭進，從尾出
雙向隊列：頭尾都可以進出

<?php
    class deque
    {
        public $queue  = array();
        public $length = 0;
        
        public function frontAdd($node){
            array_unshift($this->queue,$node);
            $this->countqueue();
        }
        
        public function frontRemove(){
            $node = array_shift($this->queue);
            $this->countqueue();
            return $node;
        }
        
        public function rearAdd($node){
            array_push($this->queue,$node);
            $this->countqueue();
        }
        
        public function rearRemove(){
            $node = array_pop($this->queue);
            $this->countqueue();
            return $node;
        }
        
        public function countqueue(){
            $this->length = count($this->queue);    
        }
    }
 ?>