[LeetCode] Add Two Numbers

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

思路：

从l1, l2头节点开始，对应位置相加并建立新节点。用一个变量carry记录进位。注意几种特殊情况：

1. 一个链表为空

l1:    NULL

l2:    1->2

sum: 1->2

2.  l1, l2长度不同，且结果有可能长度超过l1, l2中的最大长度

l1:    2->2

l2:    9->9->9

sum: 1->2->0->1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

class Solution { public: ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) { ListNode *dummy = new ListNode(0), *p = dummy; int carry = 0; while(l1 || l2 || carry) { if(l1) { carry+=l1->val; l1 = l1->next; } if(l2) { carry+=l2->val; l2 = l2->next; } p->next = new ListNode(carry%10); carry /= 10; p = p->next; } return dummy->next; } };