2. Add Two Numbers

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

处理好进位

/**
 * 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* newHead = new ListNode(0);
        ListNode* pNode = newHead;
        
        int num = 0;
        
        while(l1 != NULL || l2 != NULL){
            int val = num + (l1 ? l1->val : 0) + (l2 ? l2->val: 0);
            num = val /10;
            val = val % 10;
            pNode->next = new ListNode(val);
            pNode = pNode->next;
            if(l1 != NULL){
                l1 = l1->next;
            }
            if(l2 != NULL){
                l2 = l2->next;
            }
        }
        if(num != 0){
            pNode->next = new ListNode(num);
            pNode = pNode->next;
        }
        return newHead->next;
    }
};