单链表英语单词统计（列出所有单词及其出现次数）

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<?
/**
 * The class LinkedList allows an application to store strings in
 * alphabetical order by calling orderInsert().
 * 此处定义的 LinkedList 类，可以调用它的 方法 orderInsert(),来以字母
 * 大小的顺序储存 英文字符串。
 * The frequency for each word is also provided.
 * 同时记录 英文单词出现的次数
 * 作者： 许同春 author Tongchun Xu
 * @开源中国 Open Source, China communiity
 * 完成日期：2016年6月11日 completion date: 11 June, 2016
 */
class Node{
    public $data;
    public $frequency;
    public $next;
    function __construct($data, $next = null, $frequency = 1){
        $this->data = $data; //英文字符串
        $this->next = $next; //指向后继结点的指针
        $this->frequency=$frequency; //英文字符串出现的次数
    }
}

class LinkedList{
    private $head; //单链表的头结点，不存储数据
 function __construct(){//单链表的构造方法
    //头结点的数据为"傀儡", 不代表 任何数据
    $this->head = new Node("dummy 傀儡");
    $this->first = null;
    }

 function isEmpty(){
        return ($this->head->next == null);
    }
/*  orderInsert($data) 方法，
 *  按给定字符串 $data 的大小, 将其安插到适当的位置，
 *  以保证单链表中字符串的存储，始终是有序的。
 */
 function orderInsert($data){
    $p = new Node($data);
    if($this->isEmpty()){
        $this->head->next = $p;
    }
    else {
    $node= $this->find($data);
    if(!$node){
    $q = $this->head;
    while($q->next != NULL && strcmp($data, $q->next->data)> 0 ){
    $q = $q->next;
        }
        $p->next = $q->next;
        $q->next = $p;
    }else
    $node->frequency++;
    }
 }

 function find($value){//查询是否有给定的字符串
        $q = $this->head->next;
        while($q->next != null){
        if(strcmp($q->data,$value)==0){
                break;
            }
            $q = $q->next;
        }
        if ($q->data == $value)
        return $q;
        else
        return null;
    }

 function traversal(){//遍历单链表
        if(!$this->isEmpty()){
        $p=$this->head->next;
        echo $p->data."(".$p->frequency.") ";
        while($p->next != null){
            $p=$p->next;
            echo $p->data."(".$p->frequency.") ";
            }
        }else
        echo "链表为空！";
    }
}

$ll = new LinkedList();
$city =array("Wuhan", "Beijing", "Shanghai","Thunder Bay",
 "Tianjin", "Changsha", "Kunming", "Edmonton", "Glasgow",
 "Gongyi","Tokyo","New York","Ottawa","Moskow","Edmonton",
 "Glasgow","Edinburgh","Thunder Bay","New Delhi","Edinburgh",
  "Edmonton", "Glasgow", "Edmonton","Glasgow");
for ($i=0;$i<count($city);$i++)
$ll->orderInsert($city[$i]);
$ll->traversal();
?>