2. Add Two Numbers

• Big integer sum을 linked list 형태로 변경해서 나온 문제.

/**
 * 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 head;
        ListNode* prev = &head;
        int carry = 0;
        while (l1 != nullptr || l2 != nullptr) {
            int sum = carry;
            if (l1 != nullptr) {
                sum += l1->val;
                l1 = l1->next;
            }
            if (l2 != nullptr) {
                sum += l2->val;
                l2 = l2->next;
            }
            carry = sum >= 10? 1 : 0;
            ListNode* curr = new ListNode(sum % 10);
            prev->next = curr;
            prev = curr;
        }
        if (carry > 0) {
            prev->next = new ListNode(1);
        }
        return head.next;
    }
};