<p>You are given the <code>head</code> of a <strong>non-empty</strong> linked list representing a non-negative integer without leading zeroes.</p>

<p>Return <em>the </em><code>head</code><em> of the linked list after <strong>doubling</strong> it</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2023/05/28/example.png" style="width: 401px; height: 81px;" />
<pre>
<strong>Input:</strong> head = [1,8,9]
<strong>Output:</strong> [3,7,8]
<strong>Explanation:</strong> The figure above corresponds to the given linked list which represents the number 189. Hence, the returned linked list represents the number 189 * 2 = 378.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2023/05/28/example2.png" style="width: 401px; height: 81px;" />
<pre>
<strong>Input:</strong> head = [9,9,9]
<strong>Output:</strong> [1,9,9,8]
<strong>Explanation:</strong> The figure above corresponds to the given linked list which represents the number 999. Hence, the returned linked list reprersents the number 999 * 2 = 1998. 
</pre>

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

<ul>
	<li>The number of nodes in the list is in the range <code>[1, 10<sup>4</sup>]</code></li>
	<li><font face="monospace"><code>0 &lt;= Node.val &lt;= 9</code></font></li>
	<li>The input is generated such that the list represents a number that does not have leading zeros, except the number <code>0</code> itself.</li>
</ul>

/**
 * 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* prevNode = NULL;
        ListNode* currNode = head;

        while (currNode) {
            ListNode* nextNode = currNode -> next;
            currNode -> next = prevNode;
            prevNode = currNode;
            currNode = nextNode;
        }

        return prevNode;
    }

    ListNode* add(ListNode* &num1, ListNode* &num2, int &carry) {
        // Base Case
        if (!num1 && !num2) {
            if (carry) {
                ListNode* newDigit = new ListNode(carry);
                return newDigit;
            }
            else return NULL;
        }

        // Ek Case
        int digit1 = 0;
        int digit2 = 0;
        // Update digit1 if num1 node exists
        if (num1) {
            digit1 = num1 -> val;
            num1 = num1 -> next;
        }
        // Update digit2 if num2 node exists
        if (num2)  {
            digit2 = num2 -> val;
            num2 = num2 -> next;
        }

        cout << digit1 << ", " << digit2 << endl;
        int sum = digit1 + digit2 + carry;
        int ansDigit = sum % 10;
        carry = sum / 10;

        ListNode* ansNode = new ListNode(ansDigit);
        ansNode -> next = add(num1, num2, carry);

        return ansNode;
    }

    ListNode* doubleIt(ListNode* head) {
        head = reverse(head);
        ListNode* num1 = head;
        ListNode* num2 = num1;
        int carry = 0;
        return reverse(add(num1, num2, carry));
    }
};
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