# 2. Add Two Numbers

## 題目原文

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.
```

## **解題思路**

{% hint style="info" %}

1. 先從個位數相加一直到最高位，記得要記錄每次相加是否有進位。
   {% endhint %}

## 程式解答

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