Intersection of Sorted Posting Lists

Question

## Problem

Given multiple sorted lists of document IDs (postings), return the IDs present in all of them.

## Example
```
[1,2,4,8],[2,4,9],[2,4,5] -> [2,4]
```

## Constraints
- Lists can be large

## Approach
Multi-pointer advance on the smallest; basis of AND queries in an inverted index.