Minimum Number of Platforms in DSA C# - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to sort the arrival times array.

DSA C#

Array.[1](arrival);

Drag options to blanks, or click blank then click option'

AClear

BReverse

CSort

DCopy

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array.Reverse which reverses the array instead of sorting.

Using Array.Clear which empties the array.

Using Array.Copy which copies elements but does not sort.

We use Array.Sort to sort the arrival times so we can process trains in order.

2fill in blank

medium

Complete the code to sort the departure times array.

DSA C#

Array.[1](departure);

Drag options to blanks, or click blank then click option'

ACopy

BSort

CReverse

DClear

Attempts:

3 left

💡 Hint

Common Mistakes

Using Array.Clear which removes all elements.

Using Array.Reverse which reverses order incorrectly.

Sorting departure times helps us track when platforms get free.

3fill in blank

hard

Fix the error in the while loop condition to correctly compare arrival and departure times.

DSA C#

while (i < n && j < n && arrival[i] [1] departure[j])

Drag options to blanks, or click blank then click option'

A<

B<=

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using >= or > which reverses the logic.

Using <= which includes equality incorrectly.

We check if the next train arrives before the earliest departure to decide platform count.

4fill in blank

hard

Fill both blanks to update platform count correctly inside the loop.

DSA C#

if (arrival[i] [1] departure[j]) {
    platforms_needed[2];
    i++;
} else {
    platforms_needed--;
    j++;
}

Drag options to blanks, or click blank then click option'

A<

B++

C--

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in comparison.

Decrementing platforms_needed when a train arrives.

If arrival is before departure, we add a platform. So we use '<' and increment platforms_needed.

5fill in blank

hard

Fill all three blanks to update the maximum platforms needed.

DSA C#

max_platforms = Math.[1](max_platforms, platforms_needed[2]);

// Move to next arrival or departure
if (arrival[i] [3] departure[j]) {
    i++;
} else {
    j++;
}

Drag options to blanks, or click blank then click option'

AMax

B+

C<

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Using Math.Min instead of Math.Max.

Using '>' instead of '<' in the if condition.

Adding instead of comparing platforms_needed.

We update max_platforms with the max of current max and platforms_needed. Then move pointers based on arrival < departure.

Practice

1. What does the "Minimum Number of Platforms" problem ask you to find given train arrival and departure times?

easy

A. The total time trains spend at the station

B. The least number of platforms needed so no train waits

C. The maximum speed of any train

D. The total number of trains arriving in a day

Solution

1. Step 1: Understand the problem context

   The problem wants to find how many platforms are needed so that no train has to wait for another to leave.

2. Step 2: Identify the key output

   The output is the minimum number of platforms required to handle all trains without delay.

3. Final Answer:

   The least number of platforms needed so no train waits -> Option B

4. Quick Check:

   Minimum platforms = no waiting trains [OK]

Hint: Count overlapping trains to find minimum platforms needed [OK]

Common Mistakes:

• Confusing total trains with platforms needed
• Ignoring overlapping arrival and departure times
• Thinking about train speed or duration instead

2. Which of the following C# code snippets correctly declares arrays for arrival and departure times for the Minimum Number of Platforms problem?

easy

A. int arrival = [900, 940, 950]; int departure = [910, 1200, 1120];

B. int arrival[] = (900, 940, 950); int departure[] = (910, 1200, 1120);

C. int[] arrival = {900, 940, 950}; int[] departure = {910, 1200, 1120};

D. int[] arrival = (900, 940, 950); int[] departure = (910, 1200, 1120);

Solution

1. Step 1: Recall C# array declaration syntax

   In C#, arrays are declared with type[], and values inside curly braces {}.

2. Step 2: Check each option for correct syntax

   int[] arrival = {900, 940, 950}; int[] departure = {910, 1200, 1120}; uses int[] and curly braces {}, which is correct. Others use parentheses or wrong syntax.

3. Final Answer:

   int[] arrival = {900, 940, 950}; int[] departure = {910, 1200, 1120}; -> Option C

4. Quick Check:

   C# arrays use type[] and {} [OK]

Hint: Use int[] and curly braces {} for arrays in C# [OK]

Common Mistakes:

• Using parentheses () instead of curly braces {}
• Declaring arrays without [] after type
• Using square brackets [] for initialization

3. What is the output of the following C# code snippet that calculates minimum platforms?

int[] arrival = {900, 940, 950, 1100, 1500, 1800};
int[] departure = {910, 1200, 1120, 1130, 1900, 2000};

int platforms = CalculateMinPlatforms(arrival, departure);
Console.WriteLine(platforms);

// Assume CalculateMinPlatforms correctly implements the algorithm.

medium

A. 5

B. 4

C. 2

D. 3

Solution

1. Step 1: Sort arrival and departure times

   Arrival: 900, 940, 950, 1100, 1500, 1800 Departure: 910, 1120, 1130, 1200, 1900, 2000

2. Step 2: Count overlapping trains to find max platforms

   At 950, trains at 900, 940, 950 overlap (3 trains). Later overlaps are fewer.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Max overlap = 3 platforms [OK]

Hint: Find max overlapping intervals for minimum platforms [OK]

Common Mistakes:

• Not sorting times before counting overlaps
• Counting total trains instead of overlaps
• Mixing arrival and departure arrays

4. Identify the error in this C# code snippet for calculating minimum platforms:

int[] arrival = {900, 940, 950};
int[] departure = {910, 1200, 1120};

Array.Sort(arrival);
Array.Sort(departure);

int i = 0, j = 0, platforms = 0, maxPlatforms = 0;
while (i < arrival.Length && j < departure.Length) {
    if (arrival[i] <= departure[j]) {
        platforms++;
        i++;
    } else {
        platforms--;
        j++;
    }
    maxPlatforms = Math.Max(maxPlatforms, platforms);
}
Console.WriteLine(maxPlatforms);

medium

A. The comparison should be 'arrival[i] < departure[j]' instead of '<='

B. The loop condition should be 'i < arrival.Length || j < departure.Length'

C. The arrays should not be sorted before processing

D. The platforms variable should not be incremented

Solution

1. Step 1: Analyze the comparison operator in the loop

   Using '<=' causes counting a train as arriving exactly when another departs, increasing platforms unnecessarily.

2. Step 2: Correct comparison to '<' to avoid overlap at exact departure time

   Changing to '<' means a train arriving at the same time another departs does not increase platform count.

3. Final Answer:

   The comparison should be 'arrival[i] < departure[j]' instead of '<=' -> Option A

4. Quick Check:

   Use '<' to avoid counting simultaneous arrival and departure as overlap [OK]

Hint: Use '<' not '<=' when comparing arrival and departure [OK]

Common Mistakes:

• Using '<=' causes overcounting platforms
• Not sorting arrays before processing
• Incorrect loop condition causing missed trains

5. Given arrival times [900, 940, 950, 1100, 1500, 1800] and departure times [910, 1200, 1120, 1130, 1900, 2000], how would you modify the algorithm to also return the time intervals when the maximum number of platforms are needed?

hard

A. Track intervals when platform count equals max during iteration and store them

B. Return only the maximum platform count without intervals

C. Sort only arrival times and ignore departure times

D. Count total trains without checking overlaps

Solution

1. Step 1: Modify algorithm to track platform count changes

   While iterating, record start time when platforms reach max and end time when count drops.

2. Step 2: Store intervals where platform count equals maximum

   Keep a list of intervals with max platforms to return after processing all times.

3. Final Answer:

   Track intervals when platform count equals max during iteration and store them -> Option A

4. Quick Check:

   Record max platform intervals during count updates [OK]

Hint: Save start/end times when platform count hits max [OK]

Common Mistakes:

• Ignoring time intervals, returning only counts
• Not updating intervals when platform count changes
• Sorting only one array causing wrong intervals