# 2. Add Two Numbers

**<font color=red>难度:Medium</font>**

## 刷题内容

> 原题连接

* https://leetcode.com/problems/add-two-numbers/description/

> 内容描述

```
给定两个链表代表两个非负数，求这两个数的和
(2 -> 4 -> 3) + (5 -> 6 -> 4)
因为 342 + 465 = 807
所以返回 7 -> 0 -> 8
```

## 解题方案

> 思路


这题的的关键在于链表的数储存是倒序的，因此只要从链表头相加，再将所得数挨个储存即可，但是要注意两数相加有可能大于10要进一位。
```cpp
/**
 * 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) {
	ListNode* current;
	ListNode* ret = nullptr;
	int num = 0;
	while(l1 && l2)
	{
		int sum = l1 ->val + l2 ->val + num;
		ListNode* node = new ListNode(sum % 10);
		num = sum / 10;
		ret ? current ->next = node : ret = node;
		current = node;
		l1 = l1 ->next;
		l2 = l2 ->next;
	}
	if(l2)
		l1 = l2;
	while(l1)
	{
		int sum = num + l1 ->val;
		ListNode* node = new ListNode(sum % 10);
		num = sum / 10;
		current ->next = node;
		current = node;
		l1 = l1 ->next;
	}
	if(num)
	{
		ListNode* node = new ListNode(num);
		current ->next = node;
	}
	return ret;
	}
};
```
