[LeetCode] 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

思路：两条链表相加，注意最后如果进位不为零，要加入到结果中。使用二级指针记录节点前缀。

/**
 * 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) {
        if (l1 == NULL || l2 == NULL)
            return l1 == NULL ? l2 : l1;
            
        int carry = 0;
        ListNode *pter1 = l1;
        ListNode *pter2 = l2;
        ListNode *head = NULL;
        ListNode **prev= &head;
        while (pter1 != NULL || pter2 != NULL) {
            const int l1_val = pter1 == NULL ? 0 : pter1->val;
            const int l2_val = pter2 == NULL ? 0 : pter2->val;
            int temp_val = l1_val + l2_val + carry;
            ListNode *temp_node = new ListNode(temp_val % 10);
            carry = temp_val / 10;
            *prev = temp_node;
            prev = &(temp_node->next);
            pter1 = pter1 == NULL ? NULL : pter1->next;
            pter2 = pter2 == NULL ? NULL : pter2->next;
        }
        
        if (carry == 1) {
            ListNode *temp_node = new ListNode(1);
            *prev = temp_node;
        }
        
        return head;
    }
};