<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) {}
 * };
 */

ListNode* rev(ListNode* head){
    stack<int> s;
    ListNode* temp = head;
    while(temp!= NULL){
        s.push(temp -> val);
        temp = temp -> next;
    }
    temp= head;
    while(temp != NULL){
        temp -> val = s.top();
        s.pop();
        temp = temp -> next;
    }
    return head;
}
int length(ListNode* head){
    int cnt=0;
    while(head != NULL){
        cnt++;
        head = head -> next;
    }
    return cnt;
}
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        l1 = rev(l1);
        l2 = rev(l2);
        ListNode* temp1 = l1;
        ListNode* temp2 = l2;
        ListNode* dummy = new ListNode(0);
        ListNode* ans = dummy;

        int len1 = length(l1);
        int len2 = length(l2);
        int max =0;
        if(len1>= len2){
            max = len1;
        }else{
            max = len2;
        }
        int carry = 0;
        
        for(int i=0;i<max;i++){
            int total = 0;
            if(temp1 != NULL){
                total += temp1 -> val;
                temp1 = temp1 -> next;
            }
            if(temp2 != NULL){
                total += temp2 -> val;
                temp2 = temp2 -> next;
            }
            total += carry;
            if(total>9){
                carry =1;
                total = total -10;
            }else{
                carry = 0;
            }
            ans -> next = new ListNode(total);
            ans = ans -> next;
        }

        if(temp1 == NULL && temp2 == NULL && carry == 1){
            if(ans -> val > 9 ){
                ans -> next = new ListNode(1);
                ans -> val = ans -> val -10;
            }
            else{
                ans -> next = new ListNode(1);
            }
        }
        dummy = dummy -> next;
        dummy = rev(dummy);
        return dummy;
    }
    
};
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