<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* reverseList(ListNode* head) {
        ListNode* prev = nullptr;
        ListNode* current = head;
        ListNode* next = nullptr;

        while (current != nullptr) {
            next = current->next; 
            current->next = prev; 
            prev = current;       
            current = next;       
        }

        return prev; 
    }

    ListNode* doubleIt(ListNode* head) {
        stack<int> sumStack;
        int carry = 0;

        ListNode* temp = head;

        while (temp != NULL) {
            sumStack.push(temp->val);
            temp = temp->next;
        }

        temp = head;
        
        while (!sumStack.empty()) {
            int Sum = (2 * sumStack.top()) + carry;
            sumStack.pop();
            carry = Sum / 10;
            temp->val = Sum % 10; 
            temp = temp->next;
        }


        ListNode* newHead = reverseList(head);

        if (carry == 1) {
            ListNode* newNode = new ListNode(1);
            newNode->next = newHead;
            newHead = newNode;
        }

        return newHead;
    }
};

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