Add Two Numbers

Add Two Numbers

You are given two linked lists representing two non-negative numbers. 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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

class Solution {
public:
    ListNode *addTwoNumbers(ListNode *l1, ListNode *l2) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int carry = 0;
        ListNode *root = l1 ? l1 : l2, *node = root;
        while (l1 && l2) {
            int val = l1->val + l2->val + carry;
            if (val > 9) {
                val %= 10;
                carry = 1;
            }
            else {
                carry = 0;
            }
            node = l1;
            node->val = val;
            l1 = l1->next;
            l2 = l2->next;
        }
        
        ListNode *next = l1 ? l1 : l2;
        
        if (next) {
            node->next = next;
            node = node->next;
            
            while (carry != 0) {
                int val = node->val + carry;
                if (val > 9) {
                    node->val = val % 10;
                    carry = 1;
                    if (!node->next) {
                        node->next = new ListNode(1);
                        carry = 0;
                    }
                    else {
                        node = node->next;
                    }
                }
                else {
                    node->val = val;
                    carry = 0;
                }
            }
        }
        
        if (carry != 0) {
            node->next = new ListNode(1);
        }
        
        return root;
    }
};