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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 contains a single digit. Add the two numbers and return the sum as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example 1:

Input: l1 = [2,4,3], l2 = [5,6,4] Output: [7,0,8] Explanation: 342 + 465 = 807.

Example 2:

Input: l1 = [0], l2 = [0] Output: [0]

Example 3:

Input: l1 = [9,9,9,9,9,9,9], l2 = [9,9,9,9] Output: [8,9,9,9,0,0,0,1]

Constraints:

• The number of nodes in each linked list is in the range [1, 100].
• 0 <= Node.val <= 9
• It is guaranteed that the list represents a number that does not have leading zeros.

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int sum = 0;
        ListNode* ans = new ListNode(0);
        ListNode* cur = ans;
        while (l1 != NULL || l2 != NULL)
        {
            if (l1 != NULL)
            {
                sum += l1->val;
                l1 = l1->next;
            }
            if (l2 != NULL)
            {
                sum += l2->val;
                l2 = l2->next;
            }
            
            cur->next = new ListNode(sum % 10);
            cur = cur->next;
            
            sum /= 10;
        }
        if (sum == 1)
        {
            cur->next = new ListNode(1);
        }
        
        return ans->next;
    }
};