2. 两数相加

/**
 * 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) {
       //定义一个新链表res，一个temp的链表cur，用来当作res的指针，一个进位标志carry
        ListNode *res = new ListNode(-1);
        ListNode *cur = res;
        int carry = 0;
        while(l1 != NULL || l2 != NULL){
            //取到两个链表当前的数值
            int num1 = l1 == NULL ? 0 : l1->val;
            int num2 = l2 == NULL ? 0 : l2->val;
            //求和
            int sum = num1 + num2 + carry;
            //对进位标志的验证
            carry = sum >= 10 ? 1 : 0;
            cur->next = new ListNode(sum % 10);
            cur = cur->next;
            l1 = l1 ? l1->next : l1;
            l2 = l2 ? l2->next : l2;
        }
        if (carry == 1) {
            cur->next = new ListNode(1);
        }
        return res->next;   
    }
};