023 - Merge k Sorted Lists

Merge k sorted linked lists and return it as one sorted list. Analyze and describe its complexity.

k路归并

/**
* 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;
}

struct ListNode* mergeKLists(struct ListNode** lists, int listsSize)
{
if (listsSize == 0) return NULL;
int first = 0, second = 0, i, k;
while (listsSize > 1) {
k = 0;
for (i = 0; 2 * i < listsSize; i ++) {
first = 2 * i; second = first + 1;
if (first < listsSize - 1) {
lists[i] = mergeTwoLists(lists[first], lists[second]);
k++;
} else {
lists[i] = lists[first];
}
}
listsSize -= k;
}
return lists[0];
}