2. Add Two Numbers

题目描述：

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.

思路：逐个相加即可，注意进位。

程序:

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

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/add-two-numbers
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