PHP 查找链表倒数第i个节点

<?php
    #查找链表倒数第i个节点,倒数第0个为最后一个节点
    class Node {
        public $data = null;
        public $next = null;
    }

    #第一种方法，先算出链表总长，然后向后寻找n-i个节点
    function last_i($head, $i) {
        $cnode = $head;
        $n = 0;
        while ($cnode != null) {
            $n++;
            $cnode = $cnode->next;
        }

        $j = 1;
        $cnode = $head;
        while ($j < $n - $i) {
            $j++;
            $cnode = $cnode->next;
        }

        return $cnode;
    }

    #第二种方法，使用两个指针同时移动
    function last_i_2($head, $i) {
        $p1 = $head;
        $p2 = $head;
        $k = 0;

        while ($k < $i) {
            $p1 = $p1->next;
            $k++;
        }

        while (($p1 = $p1->next) != null) {
            $p2 = $p2->next;
        }

        return $p2;
    }

    $head = new Node();
    $n1 = new Node();
    $n2 = new Node();
    $n3 = new Node();
    $n4 = new Node();
    $head->data = 0;
    $n1->data = 1;
    $n2->data = 2;
    $n3->data = 3;
    $n4->data = 4;
    $head->next = $n1;
    $n1->next = $n2;
    $n2->next = $n3;
    $n3->next = $n4;

    echo last_i($head, 2)->data . "<br>";
    echo last_i_2($head, 2)->data;
?>

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