Minimum Number of Platforms in DSA Typescript - Build from Scratch

Minimum Number of Platforms

📖 Scenario: You are managing a small train station. You want to find out the minimum number of platforms needed so that no train has to wait.Each train has an arrival time and a departure time.

🎯 Goal: Build a program that calculates the minimum number of platforms required for the trains given their arrival and departure times.

📋 What You'll Learn

Create two arrays called arrivals and departures with exact train times.

Create a variable called n to store the number of trains.

Write logic to find the minimum number of platforms needed using the arrival and departure arrays.

Print the minimum number of platforms required.

💡 Why This Matters

🌍 Real World

Train stations and airports use this logic to manage platforms or gates so that vehicles do not wait and operations run smoothly.

💼 Career

This problem teaches how to handle overlapping intervals and resource allocation, which is useful in scheduling, event planning, and system design roles.

1

Create train arrival and departure arrays

Create an array called arrivals with these exact values: 900, 940, 950, 1100, 1500, 1800. Also create an array called departures with these exact values: 910, 1200, 1120, 1130, 1900, 2000.

DSA Typescript

// Create the arrivals and departures arrays with exact values
// Your code here

Hint

Use const arrivals = [...] and const departures = [...] with the exact numbers given.

2

Create variable for number of trains

Create a variable called n and set it to the length of the arrivals array.

DSA Typescript

const arrivals = [900, 940, 950, 1100, 1500, 1800];
const departures = [910, 1200, 1120, 1130, 1900, 2000];
// Create variable n for number of trains
// Your code here

Hint

Use const n = arrivals.length; to get the number of trains.

3

Calculate minimum number of platforms needed

Write code to calculate the minimum number of platforms needed. Sort the arrivals and departures arrays. Use two pointers i and j starting at 0. Use variables platforms_needed and max_platforms initialized to 0. Loop while i < n and j < n. If arrivals[i] <= departures[j], increment platforms_needed and i. Else decrement platforms_needed and increment j. Update max_platforms with the maximum of itself and platforms_needed inside the loop.

DSA Typescript

const arrivals = [900, 940, 950, 1100, 1500, 1800];
const departures = [910, 1200, 1120, 1130, 1900, 2000];
const n = arrivals.length;
// Calculate minimum platforms needed
// Your code here

Hint

Sort both arrays. Use two pointers and count platforms needed. Update max platforms inside the loop.

4

Print the minimum number of platforms

Print the value of max_platforms.

DSA Typescript

const arrivals = [900, 940, 950, 1100, 1500, 1800];
const departures = [910, 1200, 1120, 1130, 1900, 2000];
const n = arrivals.length;

arrivals.sort((a, b) => a - b);
departures.sort((a, b) => a - b);

let platforms_needed = 0;
let max_platforms = 0;

let i = 0;
let j = 0;

while (i  max_platforms) {
      max_platforms = platforms_needed;
    }
  } else {
    platforms_needed--;
    j++;
  }
}
// Print the minimum number of platforms
// Your code here

Hint

Use console.log(max_platforms); to print the result.

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 on the platform

B. The total number of trains arriving in a day

C. The maximum speed of trains on the platform

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

Solution

1. Step 1: Understand the problem goal

   The problem asks for the minimum number of platforms required so that no train has to wait for another to leave.

2. Step 2: Clarify what is being counted

   We count overlapping train schedules to find the peak number of trains at the station simultaneously.

3. Final Answer:

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

4. Quick Check:

   Minimum platforms = max trains overlapping [OK]

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

Common Mistakes:

• Confusing total trains with platforms needed
• Ignoring overlapping times
• Counting total arrival or departure times only

2. Which of the following TypeScript code snippets correctly sorts an array of train arrival times stored as numbers?

easy

A. arrivals.sort();

B. arrivals.sort((a, b) => a - b);

C. arrivals.sort((a, b) => b - a);

D. arrivals.sort((a, b) => a + b);

Solution

1. Step 1: Understand sorting numbers in TypeScript

   Default sort() treats elements as strings, so numeric sort needs a compare function.

2. Step 2: Identify correct compare function

   The function (a, b) => a - b sorts numbers in ascending order correctly.

3. Final Answer:

   arrivals.sort((a, b) => a - b); -> Option B

4. Quick Check:

   Numeric ascending sort uses (a, b) => a - b [OK]

Hint: Use (a, b) => a - b to sort numbers ascending [OK]

Common Mistakes:

• Using sort() without compare function for numbers
• Using (a, b) => a + b which is invalid
• Sorting descending when ascending needed

3. Given arrival times [900, 940, 950, 1100, 1500, 1800] and departure times [910, 1200, 1120, 1130, 1900, 2000], what is the minimum number of platforms needed?

medium

A. 2

B. 3

C. 4

D. 5

Solution

1. Step 1: Sort arrival and departure times

   Arrivals: 900, 940, 950, 1100, 1500, 1800; Departures: 910, 1120, 1130, 1200, 1900, 2000.

2. Step 2: Count platforms needed by comparing arrivals and departures

   At 900 arrive (1 platform), 910 depart (0), 940 arrive (1), 950 arrive (2), 1100 arrive (3), 1120 depart (2), 1130 depart (1), 1200 depart (0), 1500 arrive (1), 1800 arrive (2), 1900 depart (1), 2000 depart (0). Maximum platforms needed is 4.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Max overlapping trains = 4 [OK]

Hint: Track arrivals and departures to find max overlap [OK]

Common Mistakes:

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

4. The following TypeScript code tries to find the minimum number of platforms but has a bug. What is the bug?

function findPlatforms(arrivals: number[], departures: number[]): number {
  arrivals.sort((a, b) => a - b);
  departures.sort((a, b) => a - b);
  let platformNeeded = 1, maxPlatforms = 1;
  let i = 1, j = 0;
  while (i < arrivals.length && j < departures.length) {
    if (arrivals[i] <= departures[j]) {
      platformNeeded++;
      i++;
    } else {
      platformNeeded--;
      j++;
    }
    if (platformNeeded > maxPlatforms) maxPlatforms = platformNeeded;
  }
  return maxPlatforms;
}

medium

A. Start i from 0 instead of 1 to include first arrival

B. Use < instead of <= in comparison to avoid counting overlaps wrongly

C. Initialize platformNeeded to 0 instead of 1

D. Sort arrivals and departures in descending order

Solution

1. Step 1: Analyze loop initialization

   Variable i starts at 1, skipping the first arrival time, which causes missing counting the first train.

2. Step 2: Correct initialization for accurate counting

   Starting i at 0 ensures all arrivals are considered, giving correct platform count.

3. Final Answer:

   Start i from 0 instead of 1 to include first arrival -> Option A

4. Quick Check:

   Include all arrivals by starting i=0 [OK]

Hint: Start pointers at 0 to include all trains [OK]

Common Mistakes:

• Skipping first arrival by starting i at 1
• Changing comparison operator incorrectly
• Initializing platformNeeded wrongly

5. You have trains arriving and departing as follows: Arrivals: [1000, 1010, 1025, 1030, 1040] Departures: [1015, 1020, 1035, 1045, 1050] If you want to minimize the number of platforms by rescheduling trains without changing their arrival order, which train's departure time should you delay to reduce platform count?

hard

A. Delay the departure of train arriving at 1010

B. Delay the departure of train arriving at 1000

C. Delay the departure of train arriving at 1040

D. Delay the departure of train arriving at 1025

Solution

1. Step 1: Identify overlapping trains causing max platforms

   Trains at 1010 and 1025 overlap because 1010 departs at 1020 and 1025 arrives before that.

2. Step 2: Delay departure of train at 1010 to after 1025 departs

   Delaying 1010's departure reduces overlap with 1025, lowering platform need.

3. Final Answer:

   Delay the departure of train arriving at 1010 -> Option A

4. Quick Check:

   Delay overlapping train's departure to reduce platforms [OK]

Hint: Delay departure of earliest overlapping train to reduce platforms [OK]

Common Mistakes:

• Delaying wrong train's departure
• Changing arrival order which is fixed
• Ignoring overlap timing details