LeetCode 445 Add Two Numbers II (链表)

You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Follow up:
What if you cannot modify the input lists? In other words, reversing the lists is not allowed.

Example:

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

题目链接：https://leetcode.com/problems/add-two-numbers-ii/

三次reverse，击败100%

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    
    public ListNode reverseList(ListNode head) {
        ListNode pre = head;
        ListNode cur = head.next;
        head.next = null;
        while (cur != null) {
            ListNode nxt = cur.next;
            cur.next = pre;
            pre = cur;
            cur = nxt;
        }
        return pre;
    }
    
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        l1 = reverseList(l1);
        l2 = reverseList(l2);
        ListNode helper = new ListNode(0);
        ListNode tmp = helper;
        int extra = 0;
        while (l1 != null || l2 != null || extra != 0) {
            int val1 = l1 == null ? 0 : l1.val;
            int val2 = l2 == null ? 0 : l2.val;
            helper.next = new ListNode((val1 + val2 + extra) % 10);
            extra = (val1 + val2 + extra) / 10;
            l1 = l1 == null ? l1 : l1.next;
            l2 = l2 == null ? l2 : l2.next;
            helper = helper.next;
        }
        return reverseList(tmp.next);
    }
}