Merge K Sorted Lists

Question

## Problem

Merge `k` sorted linked lists into one sorted list.

## Example
```
[[1,4,5],[1,3,4],[2,6]] -> [1,1,2,3,4,4,5,6]
```

## Constraints
- 0 ≤ k ≤ 10^4

## Approach
Min-heap of heads -> O(N log k). Common HubSpot question.