Given the input arrivals = [900, 940, 950] and departures = [910, 1200, 1120], what is the value of max_platforms after processing the second train (index 1)?

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Greedy Algorithms - Minimum Platforms (Train Stations)

Consider the following Python function that calculates the minimum number of platforms needed. Given the input arrivals = [900, 940, 950] and departures = [910, 1200, 1120], what is the value of max_platforms after processing the second train (index 1)?

Step-by-Step Solution

Solution:

Step 1: Sort trains by arrival time
Sorted trains: [(900, 910), (940, 1200), (950, 1120)]
Step 2: Process trains up to index 1
After first train: heap=[910], max_platforms=1 Second train arrival=940, heap top=910 ≤ 940, pop 910 Push 1200, heap=[1200], max_platforms=max(1,1)=1 Since question asks after second train, max_platforms=1
Final Answer:
Option B -> Option B
Quick Check:
Heap size after second train is 1, max_platforms updated to 1 [OK]

Quick Trick: Heap pops departures ≤ arrival before push [OK]

Common Mistakes:

MISTAKES

Not popping from heap before push
Confusing max_platforms update timing
Off-by-one in iteration

Trap Explanation:

PITFALL

Candidates often forget to pop platforms freed before pushing new train departure.

Interviewer Note:

CONTEXT

Tests ability to mentally execute heap-based scheduling code.

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Given the input arrivals = [900, 940, 950] and departures = [910, 1200, 1120], what is the value of max_platforms after processing the second train (index 1)?

Step 1: Sort trains by arrival time

Step 2: Process trains up to index 1

Final Answer:

Quick Check:

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