<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* addTwoNumber(ListNode* l1, ListNode* l2) {
        int sum,carry = 0;
        ListNode* head = l1;
        ListNode* l1prev = NULL;
        ListNode* l2prev = NULL;
        while((l1 != NULL) && (l2 != NULL)){
            if(carry){
                sum = (l1->val + l2->val + carry);
                carry--;
            }
            else{
                sum = (l1->val + l2->val);
            }
            if(sum > 9){
                carry = 1;
                sum = sum % 10;
            }
            l1 -> val = sum;
            l1prev = l1;
            l1 = l1 -> next;
            l2prev = l2;
            l2 = l2 -> next;
        }
        while(l1 != NULL){
            if(carry){
                sum = (l1->val + carry);
                carry--;
            }
            else{
                sum = (l1->val);
            }
            if(sum > 9){
                carry = 1;
                sum = sum % 10;
            }
            l1 -> val = sum;
            l1prev = l1;
            l1 = l1 -> next;
        }
        if(l2 != NULL){
            l1prev -> next = l2;
        }
        while(l2 != NULL){
            if(carry){
                sum = (l2->val + carry);
                carry--;
            }
            else{
                sum = (l2->val);
            }
            if(sum > 9){
                carry = 1;
                sum = sum % 10;
            }
            l2 -> val = sum;
            l2prev = l2;
            l2 = l2 -> next;
        }
        while(carry){
            ListNode* temp = new ListNode(carry);
            ListNode* curr = head;
            while(curr -> next != NULL){
                curr = curr -> next;
            }
            curr -> next = temp;
            carry--;
        }
        return head;
    }

    ListNode* reverse(ListNode* head){
        ListNode* prev = NULL;
        ListNode* curr = head;
        ListNode* forward = NULL;
        while(curr != NULL){
            forward = curr -> next;
            curr -> next = prev;
            prev = curr;
            curr = forward;
        }
        return prev;
    }

    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        l1 = reverse(l1);
        l2 = reverse(l2);
        
        /*int sum = 0;
        int carry = 0;
        ListNode* head = l1;
        while(l1 !=  NULL && l2 != NULL){
            sum = (l1 -> val + l1 -> val + carry);
            if(sum > 9){
                sum = sum % 10;
                carry = 1;
            }
            else
                carry = 0;
            l1 -> val = sum;
            l1 = l1 -> next;
            l2 = l2 -> next;
        }
        while(l1 != NULL){
            if(carry){
                sum = l1 -> val + carry;
                if(sum > 9){
                    sum = sum % 10;
                    carry = 1;
                }
                else
                    carry = 0;
                l1 -> val = sum;
            }
            l1 = l1 -> next;
        }
        if(l2 != NULL){
            l1 -> next = l2;
        }
        while(l2 != NULL){
            if(carry){
                sum = l2 -> val + carry;
                if(sum > 9){
                    sum = sum % 10;
                    carry = 1;
                }
                else
                    carry = 0;
                l2 -> val = sum;
            }
            l2 = l2 -> next;
        }
        while(carry){

        }
        return reverse(head);
        */
        return reverse(addTwoNumber(l1, l2));
    }
};
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