Hot100【三】：2. 两数相加

// 使用虚拟节点
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode(-1);
        ListNode cur = dummy;

        int forward = 0; // 进位值
        // 1.只有有一条链表不为空，就一直做相加
        while (l1 != null || l2 != null){
            int value1 = l1 == null ? 0 : l1.val;
            int value2 = l2 == null ? 0 : l2.val;

            ListNode node = new ListNode();
            int sum = value1 + value2 + forward;
            forward = sum / 10;
            node.val = (sum % 10);
            cur.next = node;
            cur = cur.next;

            // 2.如果l1不为空，l1继续
            if (l1 != null){
                l1 = l1.next;
            }

            if (l2 != null){
                l2 = l2.next;
            }
        }

        // 3. 如果最后进位不为0
        if (forward != 0){
            ListNode node = new ListNode(forward);
            cur.next = node;
        }

        return dummy.next;

    }
}```

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