<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* prev=NULL;
        ListNode* curr=head;

        while(curr!=NULL){
            ListNode* nextNode=curr->next;
            curr->next=prev;
            prev=curr;
            curr=nextNode;
        }

        return prev;
    }


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

        ListNode* temp=head;
        while (temp != nullptr) {
            st.push(temp->val);
            temp = temp->next;
        }

        // Re-initialize temp to head to start modifying nodes
        temp = head;

        while(temp->next!=0){
            int totalSum=2*st.top() + carry;
            st.pop();
            int digit=totalSum%10;
            carry=totalSum/10;
            
            temp->val=digit;
            temp=temp->next;
        }

        //processing last node
        int totalSum=2*st.top() + carry;
        int digit=totalSum%10;
        carry=totalSum/10;
            
        temp->val=digit;

        if(carry!=0){
            ListNode *newNode=new ListNode(carry);
            temp->next=newNode;
        }
        head=reverse(head);
        return head;
    }
};

// class Solution {
// private:
//     ListNode* solve(ListNode* &temp,string& s2) {
//         ListNode* curr = temp;
//         while(curr!=nullptr) {
//             curr->val = s2[0] - '0';
//             s2.erase(0,1);
//             curr = curr->next;
//         }
//         if(s2.size()!=0) {
//             curr = temp;
//             while(curr->next!=nullptr) {
//                 curr = curr->next;
//             }
//             curr->next = new ListNode(s2[0] - '0');
//         }
//         return temp;
//     }
// public:
//     ListNode* doubleIt(ListNode* head) {
//         if(head == nullptr) {
//             return head;
//         }
//         string s1;
//         ListNode* temp = head;
//         while(temp!=nullptr) {
//             s1 += to_string(temp->val);
//             temp = temp->next;
//         }
//         int number = stoi(s1)*2;
//         string s2 = to_string(number);
//         temp = head;
//         return solve(temp,s2);
//     }
// };
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