2. Add Two Numbers

Question: 

You are given two linked lists representing two non-negative numbers. 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.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

Solution: 

 

Add each element of two list with the help of a variable "carry". When both of the lists meet their end, return result.

1. One of the list is NULL:

l1:    NULL

l2:    1->2

sum: 1->2

2.  The lengths of l1 and  l2 are different:

l1:     2->2

l2:     9->9->9

sum: 1->2->0->1

Coding(C++): 

/**
 * 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* res = new ListNode(0);
        ListNode* head = res;
        int carry = 0;
        while(l1 && l2){
            res->next = new ListNode((l1->val+l2->val+carry)%10);
            res = res->next;
            carry = (l1->val+l2->val+carry)/10;
            l1 = l1->next;
            l2 = l2->next;
        }
        while(l1){
            res->next = new ListNode((l1->val+carry)%10);
            res = res->next;
            carry = (l1->val+carry)/10;
            l1 = l1->next;
        }
        while(l2){
            res->next = new ListNode((l2->val+carry)%10);
            res = res->next;
            carry = (l2->val+carry)/10;
            l2 = l2->next;
        }
        if(carry > 0){
            res->next = new ListNode(carry);
        }
        return head->next;
    }
};