2. 两数相加

水题

设d为进位，每次两个对应位相加并加上d = tmp

新数字当前位 = tmp % 10

d = tmp / 10

注意nullptr

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *ret = new ListNode();
        ListNode *p = l1, *q = l2, *r = ret;

        int cnt = 0, d = 0;
        r->next = nullptr;
        while(p != nullptr || q != nullptr)
        {
            int tmp;
            if(p != nullptr && q != nullptr)
                tmp = p->val + q->val + d;
            else if(p != nullptr)
                tmp = p->val + d;
            else
                tmp = q->val + d;
            if(cnt++ == 0)
            {
                r->val = tmp % 10;
            }
            else
            {
                ListNode *node = new ListNode();
                node->val = tmp % 10;
                node->next = nullptr;
                r->next = node;
                r = node;
            }
            d = tmp / 10;
            if(p)
                p = p->next;
            if(q)
                q = q->next;
        }
        if(d != 0)
        {
            ListNode *node = new ListNode();
            node->val = d;
            r->next = node;
            r = node;
        }

        r->next = nullptr;

        return ret;
    }
};