按位加即可, 链表版大数和

class ListNode {
    int val;
    ListNode next;
    ListNode() {}
    ListNode(int val) { this.val = val; }
    ListNode(int val, ListNode next) { this.val = val; this.next = next; }
}
// @lc code=start
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    /**
     * 按位加即可, 链表版大数和
     */
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode dummy = new ListNode();
        ListNode cur = dummy;
        int cin = 0;
        while (l1 != null || l2 != null || cin != 0) {
            int v1 = l1 != null ? l1.val : 0;
            int v2 = l2 != null ? l2.val : 0;
            int sum = v1 + v2 + cin;
            cin = 0;
            if (sum >= 10) {
                sum -= 10;
                cin = 1;
            }
            cur.next = new ListNode(sum);
            cur = cur.next;
            l1 = l1 != null ? l1.next : null;
            l2 = l2 != null ? l2.next : null;
        }
        return dummy.next;
    }
}