21. 两两交换链表的节点

<?php
Class ListNode {
    /** @var int $val */
    public $val = 0;
    /** @var ListNode $next */
    public $next;
    public function __construct($val) {
        $this->val = $val;
    }
}
class Solution {
    /**
     * $dummy->1->2->3->4->5
     *
     * $dummy->2->1->3->4->5
     *
     * 2->1->4->3->5
     * @param ListNode $head
     * @return ListNode
     */
    public function swapPairs(ListNode $head) : ListNode {
        $dummyHead = new ListNode(0);
        $dummyHead->next = $head;
        // 变量赋值对象时，是浅拷贝（引用赋值）
        $current = $dummyHead;
        while ($current->next && $current->next->next) {
            $node1 = $current->next;
            $node2 = $node1->next;
            $nextNode = $node2->next;
            $node1->next = $nextNode;
            $node2->next = $node1;
            $current->next = $node2;
            $current = $node1;
        }
        return $dummyHead->next;
    }
}
$head = new ListNode(1);
$head->next = new ListNode(2);
$head->next->next = new ListNode(3);
$head->next->next->next = new ListNode(4);
$head->next->next->next->next = new ListNode(5);
$cls = new Solution();
$r = $cls->swapPairs($head);
print_r($r);