2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)

Output: 7 -> 0 -> 8

/** * 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) {        ListNode prehead(0),*p = &prehead;        int extra = 0 ;        while(l1 || l2|| extra){            int sum = (l1 ? l1->val : 0 )+(l2? l2->val : 0 )+extra;            p->next= new ListNode(sum%10);            extra = sum/10;            p = p->next;            l1 = l1 ? l1->next : l1;            l2 = l2 ? l2->next : l2;        }        return prehead.next;    }};