2. Add Two Numbers

這題兩年前被我做過了, 導致我一直忽略掉XD

今天再重新翻出來寫, 恩, 沒有愧對我這兩個月的狂刷

就是相加, 注意carry位

class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode *res = new ListNode(-1);
        ListNode *head = res;
        int carry = 0;
        while(l1 || l2 || carry){
            int sum = 0;
            if(l1){
                sum += l1->val;
                l1 = l1->next;
            }
            if(l2){
                sum += l2->val;
                l2 = l2->next;
            }
            sum += carry;
            carry = sum / 10;
            int r = sum % 10;
            ListNode *cur = new ListNode(r);
            head->next = cur;
            head = head->next;
        }
        return res->next;
    }
};