php 单向链表反转 reverse (没有空的头结点)

Posted 2021-01-02 mingzhanghui

* 参照php标准库设计接口

http://php.net/manual/en/class.spldoublylinkedlist.php

* 反转单向链表

reverse方法, 其他的方法为了方便测试

<?php
/**
 * Created by PhpStorm.
 * User: Mch
 * Date: 8/11/18
 * Time: 00:25
 */
class Node {
    public $value;
    public $next;
    public function __construct($data) {
        $this->value = $data;
        $this->next = null;
    }
}
 
class LinkedList implements Iterator, Countable {
    private $head;
    private $cur;
 
    public function __construct() {
        $this->head = $this->cur = null;
    }
 
    public function isEmpty() {
        return is_null($this->head);
    }
 
    public function count() {
        $p = $this->head;
        $count = 0;
        while ($p !== null) {
            $p = $p->next;
            $count++;
        }
        return $count;
    }
    public function current() {
        if ($this->isEmpty()) {
            return null;
        }
        return $this->cur->value;
    }
    public function next() {
        if (is_null($this->cur)) {
            return null;
        }
        $this->cur = $this->cur->next;
    }
    public function rewind() {
        $this->cur = $this->head;
    }
 
    public function valid() {
        return !is_null($this->cur);
    }
    public function key() {
        $p = $this->head;
        $key = 0;
        while ($p !== $this->cur) {
            $p = $p->next;
            $key++;
        }
        return $key;
    }
    public function push($value) {
        if ($this->isEmpty()) {
            $this->head = new Node($value);
            $this->cur = $this->head;
        } else {
            $this->cur = $this->head;
            while ($this->cur->next !== null) {
                $this->cur = $this->cur->next;
            }
            $this->cur->next = new Node($value);
        }
        return $this;
    }
 
    public function forEach(callable $callback, mixed $userdata = null) {
        $this->rewind();
        while($this->valid()) {
            call_user_func($callback, $this->current(), $this->key(), $userdata);
            $this->next();
        }
    }
 
    public function reverse() {
        if ($this->isEmpty())
            return;
        if ($this->count() < 2)
            return;
        $prev = null;
        $cur = $this->head;
        while ($cur) {
            // save next
            $next = $cur->next;
            // move cur to head
            $this->head = $cur;
            $cur->next = $prev;
            // iterate
            $prev = $cur;
            $cur = $next;
        }
    }
 
}

　　

* test

$list = new LinkedList();
 
for ($i = 65; $i < 91; $i++) {
    $list->push(chr($i));
}
 
$list->forEach(function($value, $index) {
    printf("[%d] => %s<br />", $index, $value);
});
echo ‘-------------------<br />‘;
$list->reverse();
$list->forEach(function($value, $index) {
    printf("[%d] => %s<br />", $index, $value);
});

　　

* output:

[0] => A
[1] => B
[2] => C
[3] => D
[4] => E
[5] => F

....

-------------------
[0] => Z
[1] => Y
[2] => X
[3] => W
[4] => V
[5] => U
...