You are given two non-empty linked lists representing two non-negative integers. 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.

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

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

Solution

  1. LinkedList的遍历。
  2. 总计的几点技巧：
    • 在遍历的过程中，要注意判断非空的情况。
    • 使用一个dummy node保存链表头。
    • 记得每一次都要将cur结点向后移动一格（此处不做会造成TLE）。
  3. 要将能提取出来的公共流程都单独提取出来。
/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int carry = 0;
        ListNode res = new ListNode(0);
        ListNode cur = res;
        while(l1 != null || l2 != null){
            int val = 0;
            if(l1 != null && l2 != null){
                val = l1.val + l2.val + carry;
                l1 = l1.next;
                l2 = l2.next;
            }else if(l1 != null){
                val = l1.val + carry;
                l1 = l1.next;
            }else{
                val = l2.val + carry;
                l2 = l2.next;
            }
            cur.next = new ListNode(val % 10);
            carry = val / 10;
            cur = cur.next;
        }
        if(carry == 1)
            cur.next = new ListNode(1);
        return res.next;
    }
}