021 - 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;
* };
*/
struct ListNode* mergeTwoLists(struct ListNode* l1, struct ListNode* l2)
{
struct ListNode *tail = NULL, *head = NULL;
if (NULL == l1 || NULL == l2) {
return l2? l2 : l1;
}
if (l1->val <= l2->val) {
head = tail = l1;
l1 = l1->next;
} else {
head = tail = l2;
l2 = l2->next;
}
tail->next = NULL;
while (l1 && l2) {
if (l1->val <= l2->val) {
tail->next = l1;
tail = tail->next;
l1 = l1->next;
} else {
tail->next = l2;
tail = tail->next;
l2 = l2->next;
}
tail->next = NULL;
}
tail->next = l1? l1:l2;
return head;
}