Merge k Sorted Lists

Merge k sorted linked lists. Given k linked lists, each sorted in ascending order, merge all the lists into one sorted linked list and return its head. The algorithm should have a time complexity of O(N log k), where N is the total number of elements across all lists. Example: Input: [[1,4,5],[1,3,4],[2,6]] Output: [1,1,2,3,4,4,5,6] Constraints: The total number of nodes across all lists is in the range [0, 10^4]. Each linked list's length is in the range [0, 500]. Node values are in the range [-10^4, 10^4]. k is in the range [1, 100]. Example Input [[1,4,5],[1,3,4],[2,6]]