<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* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode* list1=l1,*list2=l2;
        string s1,s2;

        while(list1){
            s1.push_back(list1->val+'0');
            list1=list1->next;
        }
        while(list2){
            s2.push_back(list2->val+'0');
            list2=list2->next;
        }
        int n1=s1.length(),n2=s2.length();
        int i=n1-1,j=n2-1;
        int c=0;
        string ans;
        while(i>=0 && j>=0){
            int sum=(s1[i]-'0')+(s2[j]-'0')+c;
            int r=sum%10;
            c=sum/10;
            ans=to_string(r)+ans;
            i--;j--;
        }
        while(i>=0){
            int sum=(s1[i]-'0')+c;
            int r=sum%10;
            c=sum/10;
            ans=to_string(r)+ans;
            i--;
        }
        while(j>=0){
            int sum=(s2[j]-'0')+c;
            int r=sum%10;
            c=sum/10;
            ans=to_string(r)+ans;
            j--;
        }
        if(c>0){
            ans=to_string(c)+ans;
        }
        ListNode* curr=new ListNode(-1);
        ListNode* head=curr;
        for(char c:ans){
            ListNode* node=new ListNode(c-'0');
            curr->next=node;
            curr=node;
        }
        return head->next;

    }
};
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