# 2. Add Two Numbers

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.

# Solution

Approach 1: Elementary math.

# Code (Python)

Approach 1:

# Code (C++)

Approach 1:

/**
 * 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 *head = NULL;
        ListNode *tail = NULL;
        ListNode *p1 = l1;
        ListNode *p2 = l2;
        int sum = 0;
        while (p1 != NULL || p2 != NULL)
        {
            if (p1 != NULL) sum += p1->val;
            if (p2 != NULL) sum += p2->val;            
            if (head == NULL && tail == NULL)
            {
                tail = new ListNode(sum % 10);
                head = tail;
            }
            else
            {
                tail->next = new ListNode(sum % 10);
                tail = tail->next;
            }
            sum = sum / 10;
            if (p1 != NULL) p1 = p1->next;
            if (p2 != NULL) p2 = p2->next;
        }
        if (sum > 0)
        {
            tail->next = new ListNode(sum);
            tail = tail->next;
        }
        return head;
    }
};