LeetCode之“链表”：Add Two Numbers

　　题目链接

　　题目要求：

　　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

　　这道题难度不大，具体程序如下：

/**
 * 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)
            return l2;
        else if(!l2)
            return l1;
        
        ListNode *dummy = new ListNode(0);
        ListNode *head = nullptr, *start = dummy;
        int carry = 0;
        while(l1 || l2)
        {
            if(l1 && l2)
            {
                int sum = l1->val + l2->val + carry;
                l1->val = sum % 10;
                carry = sum / 10;
                start->next = l1;
                l1 = l1->next;
                l2 = l2->next;
            }
            else if(l1)
            {
                int sum = l1->val + carry;
                l1->val = sum % 10;
                carry = sum / 10;
                start->next = l1;
                l1 = l1->next;
            }
            else
            {
                int sum = l2->val + carry;
                l2->val = sum % 10;
                carry = sum / 10;
                start->next = l2;
                l2 = l2->next;
            }
            
            start = start->next;
        }
        
        if(carry)
        {
            ListNode *node = new ListNode(1);
            start->next = node;
        }
        
        head = dummy->next;
        delete dummy;
        dummy = nullptr;
        
        return head;
    }
};