2. Add Two Numbers**

​​https://leetcode.com/problems/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.

C++ 实现 1

由于已经是从低位开始计算了, 所以不需要考虑对链表进行翻转. 使用 ​​carry_over​​ 表示进位符.

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