算法：链表实现两数之和

给出两个 非空 的链表用来表示两个非负的整数。其中，它们各自的位数是按照 逆序 的方式存储的，并且它们的每个节点只能存储 一位 数字。

如果，我们将这两个数相加起来，则会返回一个新的链表来表示它们的和。

示例：

输入：(0->2 -> 4 ->0-> 3) + (0->5 -> 6 -> 8)
输出：0->7->0->9->3
原因：30420 + 8650 =   39070

class Node
{
    public $val = null;
    public $next = null ;
    public function __construct($val)
    {
        $this->val = $val;
    }
}

function arrToList($arr){
    $head = null;
    $count = count($arr);
    $i = 0;
    while($i < $count ){
        $new = new Node($arr[$i]);
        if(empty($head)){
            $head = $new;
            $now = $head;
        }
        $now->next = $new;
        $now = $now->next;
        $i++;
    }
    return $head;
}
function addList($l1, $l2){
    $t = $l1;
    $k = 0;
    do{
    
        $val = $t->val + $l2->val + $k;
    
        $t->val = $val % 10;    
    
        $k = $val >= 10 ? 1 : 0;
    
        if (!$l2->next && !$t->next && $k) {
            $t->next = new Node(1);
            break;
        }
    
        if ($t->next && !$l2->next) {
            $l2->next = new Node(0);
        }
    
    
        if ($l2->next && !$t->next) {
            $t->next = new Node(0);
        }            
    
        $t = $t->next;
    
        $l2 = $l2->next;
    
    }while ($t);
    return $l1;
}

$arr1 = [0, 2, 4, 0, 3];
$arr2 = [0, 5, 6, 8];

$l1 = arrToList($arr1);
$l2 = arrToList($arr2);
print_r($l1);
print_r($l2);
print_r(addList($l1, $l2));

[Running] php "/Users/why/Desktop/php/why.php"
Node Object
(
    [val] => 0
    [next] => Node Object
        (
            [val] => 2
            [next] => Node Object
                (
                    [val] => 4
                    [next] => Node Object
                        (
                            [val] => 0
                            [next] => Node Object
                                (
                                    [val] => 3
                                    [next] => 
                                )

                        )

                )

        )

)
Node Object
(
    [val] => 0
    [next] => Node Object
        (
            [val] => 5
            [next] => Node Object
                (
                    [val] => 6
                    [next] => Node Object
                        (
                            [val] => 8
                            [next] => 
                        )

                )

        )

)
Node Object
(
    [val] => 0
    [next] => Node Object
        (
            [val] => 7
            [next] => Node Object
                (
                    [val] => 0
                    [next] => Node Object
                        (
                            [val] => 9
                            [next] => Node Object
                                (
                                    [val] => 3
                                    [next] => 
                                )

                        )

                )

        )

)

[Done] exited with code=0 in 0.231 seconds

解法原理：对象浅拷贝不会改变原对象的属性值，代码中$t->val是通过地址改变的l1链表的值。