<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* doubleIt(ListNode* head) {
        if (!head)
            return nullptr;

        // Step 1: Convert the linked list to a string representation of the
        // number
        string num = "";
        while (head) {
            num += to_string(head->val);
            head = head->next;
        }

        // Step 2: Double the number represented by the string
        string result = "";
        int carry = 0;
        for (int i = num.length() - 1; i >= 0; i--) {
            int digit = (num[i] - '0') * 2 + carry;
            carry = digit / 10;
            result += to_string(digit % 10);
        }
        if (carry > 0) {
            result += to_string(carry);
        }

        // Step 3: Reverse the result string as we've built it backwards
        reverse(result.begin(), result.end());

        // Step 4: Convert the result string back to a linked list
        ListNode* newHead = new ListNode(result[0] - '0');
        ListNode* current = newHead;
        for (int i = 1; i < result.length(); ++i) {
            current->next = new ListNode(result[i] - '0');
            current = current->next;
        }

        return newHead;
    }
};
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