<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) {}
 * };
 */
class Solution {
public:
    ListNode* reverse(ListNode* head){
        ListNode* prev= NULL;
        ListNode* curr= head;
        while (curr){
            ListNode* forward= curr->next;
            curr->next= prev;
            prev= curr;
            curr= forward;
        }
        return prev;
    }
    ListNode* add(ListNode* l1, ListNode* l2){
        ListNode* head= new ListNode(-1);
        ListNode* curr= head;
        int carry=0;
        while (l1!=NULL || l2!=NULL || carry>0){
            int v1=0, v2=0;
            if (l1)v1= l1->val;
            if (l2)v2= l2->val;
            int sum= v1+v2+carry;
            carry= sum/10;
            sum= sum%10;
            curr->next= new ListNode(sum);
            curr= curr->next;
            if (l1)l1= l1->next;
            if (l2)l2= l2->next;
        }
        head= head->next;
        return head;
    }
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        l1= reverse(l1);
        l2= reverse(l2);
        ListNode* ans= add(l1,l2);
        ans= reverse(ans);
        return ans;
    }
};
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