leetcode | 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.

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     struct ListNode *next;
* };
*/
//时间复杂度O(min(m, n)) 空间复杂度O(1)
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2) {
if (l1 == NULL || l2 == NULL)
return (l1 == NULL) ? l2 : l1;
struct ListNode* dummy = (struct ListNode*)malloc(sizeof(*dummy));//假节点
struct ListNode* prev = dummy;
while (l1 && l2) {
if (l1->val < l2->val) {
prev->next = l1;
l1 = l1->next;
} else {
prev->next = l2;
l2 = l2->next;
}
prev = prev->next;
}
prev->next = (l1 == NULL) ? l2 : l1;
return dummy->next;

}