HashSet for unique elements in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this HashSet code?

Consider the following C# code using HashSet<int>. What will be printed?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<int> numbers = new HashSet<int>() {1, 2, 3, 2, 4, 1};
        Console.WriteLine(numbers.Count);
    }
}

A 4

B 5

C 6

D 3

💡 Hint

HashSet stores only unique elements. Duplicate values are ignored.

The HashSet contains unique elements: 1, 2, 3, 4. Duplicates like the second 1 and 2 are ignored. So the count is 4.

❓ Predict Output

intermediate

What does this code print after adding elements?

What will be the output of this C# program?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<string> fruits = new HashSet<string>();
        fruits.Add("apple");
        fruits.Add("banana");
        fruits.Add("apple");
        fruits.Add("orange");
        Console.WriteLine(fruits.Count);
    }
}

A 4

B 3

C 2

D 1

💡 Hint

HashSet ignores duplicates when adding.

"apple" is added twice but only stored once. The unique fruits are "apple", "banana", and "orange". Count is 3.

🔧 Debug

advanced

Why does this HashSet code throw an exception?

Examine the code below. It throws a runtime exception. What is the cause?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<int> set = new HashSet<int>() {1, 2, 3};
        foreach (var item in set) {
            if (item == 2) {
                set.Remove(item);
            }
        }
    }
}

A The HashSet is empty, so Remove fails.

B HashSet does not support Remove method.

C The foreach loop variable 'item' is read-only and cannot be used in Remove.

D Modifying a collection while iterating causes an InvalidOperationException.

💡 Hint

Think about what happens if you change a collection while looping over it.

Removing items from a HashSet during foreach iteration causes an InvalidOperationException because the collection is modified during enumeration.

❓ Predict Output

advanced

What is the output of this HashSet intersection code?

What will this program print?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        HashSet<int> setA = new HashSet<int>() {1, 2, 3, 4};
        HashSet<int> setB = new HashSet<int>() {3, 4, 5, 6};
        setA.IntersectWith(setB);
        Console.WriteLine(string.Join(",", setA));
    }
}

A 5,6

B 1,2

C 3,4

D 1,2,3,4,5,6

💡 Hint

IntersectWith keeps only elements present in both sets.

After intersection, setA contains only elements common to setA and setB: 3 and 4.

🧠 Conceptual

expert

What is the time complexity of HashSet operations?

Which statement best describes the average time complexity of Add, Remove, and Contains operations in a C# HashSet<T>?

A Add, Remove, and Contains operations run on average in O(1) time due to hashing.

B Add and Remove are O(1), but Contains is O(n) because it searches linearly.

C All operations run in O(n) time because HashSet uses a list internally.

D All operations run in O(log n) time because HashSet uses a balanced tree.

💡 Hint

Think about how hashing helps find elements quickly.

HashSet uses a hash table internally, so Add, Remove, and Contains usually run in constant time O(1) on average.