<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) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbersHelper(ListNode* l1, ListNode* l2, int diff) {
        if(l1 -> next != NULL && l2 -> next != NULL) {
            cout << l1 -> val << " " << l2-> val << " " << diff << "\n";
            if(diff > 0) {
                ListNode* node = addTwoNumbersHelper(l1 -> next, l2, diff - 1);
                ListNode* newNode = new ListNode(l1 -> val + (node -> val / 10), node);
                return newNode;
            }
            else{
                ListNode* node = addTwoNumbersHelper(l1 -> next, l2 -> next, diff - 1);
                ListNode* newNode = new ListNode(l1 -> val + l2 -> val + (node -> val / 10), node);
                return newNode;
            }
        }
        else if(l1 -> next != NULL && l2 -> next == NULL){
            ListNode* node = addTwoNumbersHelper(l1 -> next, l2, diff - 1);
            ListNode* newNode = new ListNode(l1 -> val + (node -> val / 10), node);
            return newNode;
        }
        return new ListNode(l1 -> val + l2 -> val);
    }
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *it1 = l1;
        ListNode *it2 = l2;
        int size1 = 0;
        int size2 = 0;
        while(it1 != NULL || it2 != NULL) {
            if(it1 != NULL) {
                size1++;
                it1 = it1 -> next;
            }
            if(it2 != NULL) {
                size2++;
                it2 = it2 -> next;
            }
        }
        // cout << size1 << " " << size2 << "\n"; 
        if(size1 < size2) {
            swap(l1, l2);
        }
        
        int d = abs(size1 - size2);
        int diff = d;
        // cout << diff << "\n";
        ListNode* node = addTwoNumbersHelper(l1, l2, diff);
        if(node -> val >= 10) {
            ListNode* newNode = new ListNode(node -> val / 10, node);
            node = newNode;
        }
        ListNode* it = node;
        while(it != NULL) {
            it -> val %= 10;
            it = it -> next;
        }
        return node;
    }
};
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