Why Recursion Exists and What Loops Cannot Express Cleanly in DSA C# - Test Your Knowledge

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the recursive call for factorial calculation.

DSA C#

int Factorial(int n) {
    if (n <= 1) return 1;
    return n * Factorial([1]);
}

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

An + 1

Bn - 1

C1

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n causes infinite recursion.

The factorial of n is n multiplied by factorial of (n-1). So the recursive call must be Factorial(n - 1).

2fill in blank

medium

Complete the code to print numbers from n down to 1 using recursion.

DSA C#

void PrintDescending(int n) {
    if (n == 0) return;
    Console.WriteLine(n);
    PrintDescending([1]);
}

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

An - 1

B0

C1

Dn + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using 1 causes infinite loop.

To print numbers from n down to 1, we reduce n by 1 in each recursive call.

3fill in blank

hard

Complete the recursive calls for the Fibonacci function.

DSA C#

int Fibonacci(int n) {
    if (n <= 1) return n;
    return Fibonacci([1]) + Fibonacci([2]);
}

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

An + 1

Bn - 2

Cn - 1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same argument for both recursive calls.

Using n + 1 causes infinite recursion.

The nth Fibonacci number is the sum of the two preceding ones: Fibonacci(n) = Fibonacci(n-1) + Fibonacci(n-2).

4fill in blank

hard

Fill both blanks to create a LINQ expression that builds a dictionary mapping words to their lengths for words longer than 3 characters.

DSA C#

var lengths = words.Where(word => word.Length > 3).ToDictionary([1], [2]);

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

A"word"

Bword.Length

Cword

Dword.Length > 3

Attempts:

3 left

💡 Hint

Common Mistakes

Using string literals as keys instead of variables.

Using boolean expressions as values.

The dictionary maps each word (key) to its length (value). So the key is 'word' and the value is 'word.Length'.

5fill in blank

hard

Fill all three blanks to build a dictionary that maps uppercase words to their lengths for words longer than 3 characters.

DSA C#

var result = new Dictionary<string, int>();
foreach (var word in words) {
    if (word.Length > [1]) {
        result[[2]] = [3];
    }
}

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

A3

Bword.ToUpper()

Cword.Length

D4

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong length threshold.

Using original word instead of uppercase as key.

The condition checks if word length is greater than 3. The key is the uppercase word, and the value is the word's length.

Practice

1. Why do we use recursion instead of loops in some problems?

easy

A. Because loops cannot repeat tasks

B. Because loops are slower than recursion in all cases

C. Because recursion uses less memory than loops

D. Because recursion can express problems with self-similar structure more clearly

Solution

1. Step 1: Understand problem structure

   Some problems have parts that look like smaller versions of themselves, like a tree or factorial calculation.

2. Step 2: Compare recursion and loops

   Loops repeat tasks but cannot naturally express self-similar or nested structures clearly, while recursion can call itself to handle these.

3. Final Answer:

   Because recursion can express problems with self-similar structure more clearly -> Option D

4. Quick Check:

   Recursion = self-similar clarity [OK]

Hint: Recursion fits problems with repeated smaller versions [OK]

Common Mistakes:

• Thinking loops are always faster
• Believing recursion always uses less memory
• Assuming loops cannot repeat tasks

2. Which of the following is the correct syntax to define a recursive method in C#?

easy

A. void RecursiveMethod() { RecursiveMethod(); }

B. void RecursiveMethod() { while(true) { RecursiveMethod(); } }

C. void RecursiveMethod() { if(condition) RecursiveMethod(); else return; }

D. void RecursiveMethod() { for(int i=0; i<10; i++) RecursiveMethod(); }

Solution

1. Step 1: Identify base case in recursion

   Recursive methods must have a condition to stop calling themselves to avoid infinite loops.

2. Step 2: Check options for base case

   Only void RecursiveMethod() { if(condition) RecursiveMethod(); else return; } has a condition that stops recursion with an else return statement.

3. Final Answer:

   void RecursiveMethod() { if(condition) RecursiveMethod(); else return; } -> Option C

4. Quick Check:

   Base case needed = void RecursiveMethod() { if(condition) RecursiveMethod(); else return; } [OK]

Hint: Look for base case to stop recursion [OK]

Common Mistakes:

• Missing base case causing infinite recursion
• Using loops inside recursion incorrectly
• Confusing recursion with infinite loops

3. What is the output of this C# recursive function?

int Sum(int n) {
  if (n == 0) return 0;
  return n + Sum(n - 1);
}
Console.WriteLine(Sum(3));

medium

A. 6

B. 3

C. 0

D. StackOverflowException

Solution

1. Step 1: Trace recursive calls

   Sum(3) calls 3 + Sum(2), Sum(2) calls 2 + Sum(1), Sum(1) calls 1 + Sum(0), Sum(0) returns 0.

2. Step 2: Calculate return values

   Sum(1) = 1 + 0 = 1, Sum(2) = 2 + 1 = 3, Sum(3) = 3 + 3 = 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum 1+2+3 = 6 [OK]

Hint: Add numbers down to zero recursively [OK]

Common Mistakes:

• Confusing return values order
• Forgetting base case returns 0
• Expecting runtime error

4. Identify the error in this recursive C# function that calculates factorial:

int Factorial(int n) {
  if (n == 0) return 1;
  return n * Factorial(n);
}

medium

A. Recursive call uses same argument causing infinite recursion

B. Multiplication operator is incorrect

C. Factorial of 0 should return 0

D. Missing base case for n == 1

Solution

1. Step 1: Analyze recursive call argument

   The function calls Factorial(n) again without changing n, so it never reaches base case.

2. Step 2: Identify infinite recursion cause

   Because n does not decrease, the function calls itself endlessly causing a stack overflow.

3. Final Answer:

   Recursive call uses same argument causing infinite recursion -> Option A

4. Quick Check:

   Argument must change to reach base case [OK]

Hint: Check if recursive call changes input parameter [OK]

Common Mistakes:

• Thinking base case is missing
• Believing factorial(0) returns 0
• Ignoring infinite recursion risk

5. You want to print all paths from root to leaves in a tree structure. Why is recursion preferred over loops for this task?

hard

A. Because loops cannot access tree nodes

B. Because recursion naturally handles branching and backtracking in trees

C. Because recursion uses less CPU than loops

D. Because loops cannot print strings

Solution

1. Step 1: Understand tree traversal needs

   Printing all root-to-leaf paths requires exploring branches and returning to previous nodes (backtracking).

2. Step 2: Compare recursion and loops for trees

   Recursion calls itself for each branch and naturally backtracks when returning, while loops need complex stacks to simulate this.

3. Final Answer:

   Because recursion naturally handles branching and backtracking in trees -> Option B

4. Quick Check:

   Recursion = easy tree traversal [OK]

Hint: Recursion fits branching structures like trees [OK]

Common Mistakes:

• Thinking loops cannot access nodes
• Assuming recursion is always faster
• Believing loops cannot print strings