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

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题意：链表形式的加法

思路：比较像大数加法吧，照着从左到右模拟过去

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
      if (l2 == NULL) return l1;
		if (l1 == NULL) return l2;
		ListNode *l3 = NULL;
		ListNode *p = NULL;
		int flag = 0;
		while (l1 != NULL || l2 != NULL) {
			int sum = flag;
			if (l1 != NULL) {
				sum += l1->val;
				l1 = l1->next;
			}
			if (l2 != NULL) {
				sum += l2->val;
				l2 = l2->next;
			}
			int cur = sum % 10;
			flag = sum / 10;
			ListNode *tmp = new ListNode(cur);

			if (l3 == NULL) {
				l3 = tmp;
				p = l3;
			} else {
				p->next = tmp;
				p = p->next;
			}
		}

		if (flag != 0) {
			ListNode *tmp = new ListNode(flag);
			p->next = tmp;
		}

		return l3;  
    }
};