https://leetcode.com/problems/add-two-numbers/description/
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.
We have l1=343, l2=261 when add(l1,l2) we have 3+2=5, 4+6=10 /10=1 1 carry to next,
second digit set to (10%10=0) 0, 3+1=4 but 4 + carry = 5, add(l1,l2) = 505
1 public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
2 ListNode dummy = new ListNode(0);
3 int sum =0 ;
4 ListNode curr = dummy ;
5 ListNode p1 = l1, p2 = l2 ;
6 while (p1 != null || p2 != null){
7 if (p1 != null){
8 sum += p1.val ;
9 p1 = p1.next ;
10 }
11 if (p2 != null){
12 sum += p2.val ;
13 p2 = p2.next;
14 }
15 curr.next = new ListNode(sum %10) ; //10 %10 =0
16 sum /=10 ; //7/10 =0, 10/10 =1 11/10 =1
17 curr = curr.next ;
18 }
19 /* 642 + 465
20 * if (2 -> 4 -> 6) + (5 -> 6 -> 4)
21 * 2+5 = 7 node:7
22 * 4+ 6 = 10, node: 0 carry 1
23 * 6+4+1 = 11 node: 1 carry 1 === this needs special treatment
24 * 链表连成： 7011 系统自动转换成： 1107
25 * */
26 //两位数想加 最大进1位 不可能最后成2
27 if (sum == 1){
28 curr.next = new ListNode(1) ;
29 }
30 return dummy.next ;
31 }
