PHP使用链表实现两数相加

实例：

输入：l1 = [2,4,3], l2 = [5,6,4]
输出：[7,0,8]
解释：342 + 465 = 807.

代码：

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {

    /**
     * @param ListNode $l1
     * @param ListNode $l2
     * @return ListNode
     */
    function addTwoNumbers($l1, $l2) {
       $obj = null;
       $carry = 0; 
       do{
        $val = $l1->val + $l2->val + $carry;
        if($val < 10){
            $carry = 0;
        }else {
            $carry = 1;
            $val -= 10;
        }

        if($obj === null){
            $obj = new ListNode($val);
            $cur = $obj;
        }else{
            $cur->next = new ListNode($val);
            $cur = $cur->next;
        }
        $l1 = $l1->next;
        $l2 = $l2->next;
       }while($l1 || $l2 || $carry);
       return $obj;
    }
}

优化：

/**
 * Definition for a singly-linked list.
 * class ListNode {
 *     public $val = 0;
 *     public $next = null;
 *     function __construct($val = 0, $next = null) {
 *         $this->val = $val;
 *         $this->next = $next;
 *     }
 * }
 */
class Solution {

    /**
     * @param ListNode $l1
     * @param ListNode $l2
     * @return ListNode
     */
    function addTwoNumbers($l1, $l2) {
       $obj = null;
       $carry = 0; 
       do{
        $val = $l1->val + $l2->val + $carry;
        if($val < 10){
            $carry = 0;
        }else {
            $carry = 1;
            $val -= 10;
        }

        $tmp_obj = new ListNode($val);
        if($obj === null){
            $obj = $tmp_obj;
        }else{
            $cur->next = $tmp_obj; // 将新的节点挂到当前节点之后
        }
        $cur = $tmp_obj;    // 注意对象的clone和=的使用
        $l1 = $l1->next;
        $l2 = $l2->next;
       }while($l1 || $l2 || $carry);
       return $obj;
    }
}

方法2种精简了方法1的写法，注意对象中clone和=的使用，以及链表中定位下一个节点的写法