Merge K Sorted Lists

lists holds k linked lists, each already sorted in ascending order.
Combine them into a single linked list that is also sorted in ascending order, and return its head.

Example:

• Input: lists = [[1,4,5],[1,3,4],[2,6]]
• Output: [1,1,2,3,4,4,5,6]

Min-Heap Approach (O(N log k))
Instead of comparing the heads of all k lists at every step (which takes O(k)), we can use a Min-Heap.

1. Initialize: Push the head node of every non-empty list into a Min-Heap.
2. Iterate: While heap is not empty:
   • Pop the smallest node (root) from heap.
   • Add this node to our result list.
   • If the popped node has a next node, push that next node into the heap.
3. Result: The result list is sorted.

The k lists as chains, the min-heap holding the front node of each, and the result growing from a dummy node. Each pop links the smallest node into the result and pushes the node after it.