21. Merge Two Sorted Lists

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

AC code:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode t2(INT_MIN);
        ListNode* test = &t2;
        while (l1 && l2) {
            if (l1->val < l2->val) {
                test->next = l1;
                l1 = l1->next;
            } else {
                test->next = l2;
                l2 = l2->next;    
            }
            test = test->next;
        }
        test->next = l1 ? l1 : l2;
        return t2.next;
    }
};

Runtime: 8 ms, faster than 90.09% of C++ online submissions for Merge Two Sorted Lists.