Add Two Numbers leetcode

You are given two linked lists representing two non-negative numbers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Example

Given 7->1->6 + 5->9->2. That is, 617 + 295.

Return 2->1->9. That is 912.

Given 3->1->5 and 5->9->2, return 8->0->8.

维护一个carry用于进位 因为是反过来写的所以进位在后面 可以直接加
时间复杂度O(n)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
public class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
       if (l1 == null) {
           return l2;
       }
       if (l2 == null) {
           return l1;
       }
       int carry = 0;
       ListNode head = new ListNode(-1);
       ListNode l3 = head;
       while (l1 != null || l2 != null) {
           if (l1 != null) {
               carry += l1.val;
               l1 = l1.next;
           }
           if (l2 != null) {
               carry += l2.val;
               l2 = l2.next;
           }
           l3.next = new ListNode(carry%10);
           carry = carry/10;
           l3 = l3.next;
       }
       if (carry > 0) {
           l3.next = new ListNode(carry);
       }
       return head.next;
    }
}