Add Two Numbers.cpp

/*
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 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.
*/

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */

class Solution
{
public:
  ListNode *addTwoNumbers(ListNode *l1, ListNode *l2)
  {
    ListNode *dummy = new ListNode(-1);
    auto temp = dummy;
    int carry = 0;

    while (l1 && l2)
    {
      int digit = l1->val + l2->val + carry;
      carry = digit / 10;
      digit = digit % 10;
      l1 = l1->next;
      l2 = l2->next;
      ListNode *newNode = new ListNode(digit);
      temp->next = newNode;
      temp = temp->next;
    }

    while (l1)
    {
      int digit = l1->val + carry;
      carry = digit / 10;
      digit = digit % 10;
      l1 = l1->next;
      ListNode *newNode = new ListNode(digit);
      temp->next = newNode;
      temp = temp->next;
    }

    while (l2)
    {
      int digit = l2->val + carry;
      carry = digit / 10;
      digit = digit % 10;
      l2 = l2->next;
      ListNode *newNode = new ListNode(digit);
      temp->next = newNode;
      temp = temp->next;
    }

    while (carry)
    {
      int digit = carry;
      carry = digit / 10;
      digit = digit % 10;
      ListNode *newNode = new ListNode(digit);
      temp->next = newNode;
      temp = temp->next;
    }

    return dummy->next;
  }
};