<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* reverse(ListNode* head)
    {
        ListNode* curr = head;
        ListNode* prev = NULL;
        ListNode* fwd = NULL;

        while(curr != NULL)
        {
            fwd = curr -> next;
            curr -> next = prev;
            prev = curr;
            curr = fwd;
        }
        return prev;
    }

    void insertAtTail(ListNode* &head, ListNode* &tail, int data)
    {
        ListNode* temp = new ListNode(data);
        //empty list
        if(head == NULL)
        {
            head = temp;
            tail = temp;
            return ;
        } else {
            tail -> next = temp;
            tail = temp;
        }
    }

    ListNode* add(ListNode* l1, ListNode* l2)
    {
        int carry = 0;
        ListNode* ansHead = NULL;
        ListNode* ansTail = NULL;

        while(l1 != NULL || l2 != NULL || carry != 0)
        {
            int val1 = 0, val2 = 0;
            if(l1 != NULL)
            {
                val1 = l1 -> val;
            }

            if(l2 != NULL)
            {
                val2 = l2 -> val;
            }

            int sum = carry + val1 + val2;
            int digit = sum % 10;

            insertAtTail(ansHead, ansTail, digit);
            carry = sum / 10;

            if(l1 != NULL)
            {
                l1 = l1 -> next;
            }   
            
            if(l2 != NULL)
            {
                l2 = l2 -> next; 
            }  
        }

        return ansHead;
    }

    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) 
    {
        //Step 1 - Reverse the lists
        l1 = reverse(l1);
        l2 = reverse(l2);

        //Step 2 - Adding the numbers
        ListNode* ans = add(l1, l2);

        //Step 3 - Reverse the list again
        ans = reverse(ans);

        return ans;
    }
};
77200