LeetCode 21: Merge Two Sorted Lists

/**
 * 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* mergeTwoLists(ListNode* l1, ListNode* l2) {
		if (!l1) return l2;
		if (!l2) return l1;

		if (l1->val < l2->val) {
			l1->next = mergeTwoLists(l1->next, l2);
			return l1;
		}
		else {
			l2->next = mergeTwoLists(l1, l2->next);
			return l2;
		}
	}
};