<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* rev(ListNode* head)
    {
        if(head->next==NULL)
        return head;

        ListNode* ptr1=head;
        ListNode* ptr2=ptr1->next;
        ListNode* ptr3=ptr2->next;

        while(ptr2!=NULL)
        {
            ptr2->next=ptr1;
            ptr1=ptr2;
            ptr2=ptr3;
            if(ptr2==NULL)
            break;
            ptr3=ptr2->next;
        }
        head->next=NULL;
        head=ptr1;

        return head;
    }
    int add(vector<int> &result,int a,int b,int c)
    {
        int ans=a+b+c;
        result.push_back(ans%10);
        return ans/10;
    }
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {

        if(l1->next!=NULL)
        l1=rev(l1);
        if(l2->next!=NULL);
        l2=rev(l2);

        vector<int> result;
        int carry=0;

        while(l1!=NULL && l2!=NULL)
        {
            carry=add(result,l1->val,l2->val,carry);
            l1=l1->next;
            l2=l2->next;
        }

        while(l1!=NULL)
        {
            carry=add(result,l1->val,0,carry);
            l1=l1->next;
        }

        while(l2!=NULL)
        {
            carry=add(result,0,l2->val,carry);
            l2=l2->next;
        }
        if(carry!=0)
        result.push_back(carry);
        
        reverse(result.begin(),result.end());

        ListNode* head=new ListNode(result[0]);
        head->next=NULL;
        ListNode *ptr=head;
        for(int i=1;i<result.size();i++)
        {
            ListNode *temp = new ListNode(result[i]);
            ptr->next=temp;
            temp->next=NULL;
            ptr=ptr->next;
        }

        return head;
        
    }
};
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