Given arrays of train arrival times `arrivals` and departure times `departures` (both in 24-hour format, e.g., 900 for 9:00 AM), compute the minimum number of platforms required so that no train waits. **Pattern focus:** Separate Start and End Sweeps. Similar to meeting rooms, count concurrent trains.
Example 1:
Input:
arrivals = [900,940,950,1100,1500,1800] departures = [910,1200,1120,1130,1900,2000]
Output:
3
Explanation:
Three trains overlap around 1100 hours.
Example 2:
Input:
arrivals = [100,110,120] departures = [101,111,121]
Output:
1
Explanation:
Trains do not overlap; 1 platform suffices.