2.Add Two Numbers (List)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        int sum = l1->val + l2->val; 
        int carry = sum/10;
        ListNode* root = new ListNode(sum%10);
        ListNode* current = root;
        
        l1 = l1->next;
        l2 = l2->next;
        while(l1 && l2){
            sum = l1->val + l2->val + carry;
            carry = sum/10;
            current->next = new ListNode(sum%10);
            current = current->next;
            l1 = l1->next;
            l2 = l2->next;
        }
        
        while(l1){
            sum = l1->val+ carry;
            carry = sum/10;
            current->next = new ListNode(sum%10);
            current = current->next; 
            l1 = l1->next;
        }
        
        while(l2){
            sum = l2->val+ carry;
            carry = sum/10;
            current->next = new ListNode(sum%10);
            current = current->next; 
            l2 = l2->next;
        }
        
        if(carry){
            current->next = new ListNode(carry);
        }
        return root;
    }
};