Minimum Number of Platforms

Question

## Problem

Given train arrival and departure times, find the minimum number of platforms needed so no train waits.

## Example
```
arr=[900,940,950,1100], dep=[910,1200,1120,1130] -> 3
```

## Constraints
- 1 ≤ n ≤ 10^5

## Approach
Sort arrivals and departures; sweep with two pointers tracking concurrent trains. Reported ServiceNow question.