Merge K Sorted Lists

Question

## Problem

Merge k sorted lists into one (analogous to merging postings lists in search).

## 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).