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];
}