<p>You are given two <strong>non-empty</strong> linked lists representing two non-negative integers. The most significant digit comes first and each of their nodes contains a single digit. Add the two numbers and return the sum as a linked list.</p>

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

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/04/09/sumii-linked-list.jpg" style="width: 523px; height: 342px;" />
<pre>
<strong>Input:</strong> l1 = [7,2,4,3], l2 = [5,6,4]
<strong>Output:</strong> [7,8,0,7]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> l1 = [2,4,3], l2 = [5,6,4]
<strong>Output:</strong> [8,0,7]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> l1 = [0], l2 = [0]
<strong>Output:</strong> [0]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in each linked list is in the range <code>[1, 100]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 9</code></li>
	<li>It is guaranteed that the list represents a number that does not have leading zeros.</li>
</ul>

<p>&nbsp;</p>
<p><strong>Follow up:</strong>&nbsp;Could you solve it without reversing the input lists?</p>

/**
 * 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) {}
 * };
 */
#define ll long long
class Solution {
public:
    ListNode* solve(ListNode* l1, ListNode* l2) {
        ListNode* f = NULL;
        ListNode* l = NULL;
        ll sum = 0;
        while(l1 != NULL and l2 != NULL){
            sum = sum + (l1 -> val) + (l2 -> val);
            ListNode * newNode = new ListNode(sum % 10);
            newNode -> next = NULL;
            sum /= 10;
            if(f == NULL){
                f = newNode ;
                l = newNode;
            }
            else {
                newNode -> next = f ;
                f = newNode;
            }
            l1 = l1 -> next;
            l2 = l2 -> next;
        }
        while(l1 != NULL){
          sum += l1 -> val;  
          ListNode * newNode = new ListNode(sum % 10);
          newNode -> next = NULL;
          sum /= 10;
          if(f == NULL){
                f = newNode ;
                l = newNode;
            }
          else {
                newNode -> next = f ;
                f = newNode;
            }
            l1 = l1 -> next;
        }
        while(l2 != NULL){
          sum += l2 -> val;  
          ListNode * newNode = new ListNode(sum % 10);
          newNode -> next = NULL;
          sum /= 10;
          if(f == NULL){
                f = newNode ;
                l = newNode;
            }
          else {
                newNode -> next = f ;
                f = newNode;
            }
            l2 = l2 -> next;
        }
        while(sum){
          ListNode * newNode = new ListNode(sum % 10);
          newNode -> next = NULL;
          sum /= 10;
          if(l == NULL){
                f = newNode ;
                l = newNode;
            }
          else {
                newNode -> next = f ;
                f = newNode;
            }
        }
        return f;
    }
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode * d1 = NULL , *d2 = NULL;
        while(l1 != NULL){
            ListNode* newNode = new ListNode(l1 -> val);
            newNode -> next = NULL;
            if(d2 == NULL){
                d2 = newNode;
                d1 = newNode;
            }
            else {
                newNode -> next = d1;
                d1 = newNode;
            }
            l1 = l1 -> next ;
        }
        ListNode * c1 = NULL , *c2 = NULL;
        while(l2 != NULL){
            ListNode* newNode = new ListNode(l2 -> val);
            newNode -> next = NULL;
            if(c2 == NULL){
                c2 = newNode;
                c1 = newNode;
            }
            else {
                newNode -> next = c1;
                c1 = newNode;
            }
            l2 = l2 -> next ;
        }
        return solve(c1 , d1);;
    }
};
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