Recursion on Arrays and Strings in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Sum of Array Elements

What is the output of the following C# code that recursively sums elements of an array?

DSA C#

using System;

class Program {
    static int RecursiveSum(int[] arr, int index) {
        if (index == arr.Length) return 0;
        return arr[index] + RecursiveSum(arr, index + 1);
    }

    static void Main() {
        int[] numbers = {1, 2, 3, 4};
        Console.WriteLine(RecursiveSum(numbers, 0));
    }
}

A 0

B 24

C 10

D Throws an exception

💡 Hint

Think about how the function adds each element starting from index 0 until the end.

The function adds each element of the array starting from index 0 recursively until it reaches the end, summing 1+2+3+4 = 10.

❓ Predict Output

intermediate

Output of Recursive String Reverse

What is the output of this C# code that reverses a string recursively?

DSA C#

using System;

class Program {
    static string ReverseString(string s) {
        if (s.Length <= 1) return s;
        return ReverseString(s.Substring(1)) + s[0];
    }

    static void Main() {
        string input = "code";
        Console.WriteLine(ReverseString(input));
    }
}

A edoc

B code

C Throws an exception

D edoced

💡 Hint

The function takes the first character and puts it at the end after reversing the rest.

The function reverses the string by recursively reversing the substring from index 1 and appending the first character at the end.

❓ Predict Output

advanced

Output of Recursive Palindrome Check

What is the output of this C# code that checks if a string is a palindrome using recursion?

DSA C#

using System;

class Program {
    static bool IsPalindrome(string s, int start, int end) {
        if (start >= end) return true;
        if (s[start] != s[end]) return false;
        return IsPalindrome(s, start + 1, end - 1);
    }

    static void Main() {
        string test = "racecar";
        Console.WriteLine(IsPalindrome(test, 0, test.Length - 1));
    }
}

A True

B False

C Throws an exception

D racecar

💡 Hint

A palindrome reads the same forwards and backwards.

The function compares characters from start and end moving inward. Since "racecar" is a palindrome, it returns true.

❓ Predict Output

advanced

Output of Recursive Fibonacci with Memoization

What is the output of this C# code that calculates the 5th Fibonacci number using recursion with memoization?

DSA C#

using System;
using System.Collections.Generic;

class Program {
    static Dictionary<int, int> memo = new Dictionary<int, int>();

    static int Fibonacci(int n) {
        if (n <= 1) return n;
        if (memo.ContainsKey(n)) return memo[n];
        memo[n] = Fibonacci(n - 1) + Fibonacci(n - 2);
        return memo[n];
    }

    static void Main() {
        Console.WriteLine(Fibonacci(5));
    }
}

A 3

B 8

C Throws an exception

D 5

💡 Hint

Remember Fibonacci sequence starts 0,1,1,2,3,5,...

The 5th Fibonacci number (0-based) is 5. Memoization avoids repeated calculations.

🧠 Conceptual

expert

Maximum Call Stack Depth in Recursive String Palindrome Check

Consider a recursive function that checks if a string of length n is a palindrome by comparing characters from the ends moving inward. What is the maximum depth of the call stack during execution?

A n

B n/2

C log n

D 1

💡 Hint

Each recursive call moves inward by one character from both ends.

Each recursive call reduces the problem size by 2 characters (one from start and one from end), so maximum depth is about n/2.

Practice

1. What is the main idea behind using recursion on arrays or strings?

easy

A. Sorting the array or string before processing

B. Using loops to iterate over each element

C. Breaking the problem into smaller similar problems until a base case is reached

D. Storing all elements in a new data structure

Solution

1. Step 1: Understand recursion concept

   Recursion solves problems by calling itself with smaller inputs until a simple base case is reached.

2. Step 2: Apply to arrays or strings

   For arrays or strings, recursion processes one element and then calls itself on the rest.

3. Final Answer:

   Breaking the problem into smaller similar problems until a base case is reached -> Option C

4. Quick Check:

   Recursion = divide and conquer [OK]

Hint: Think: solve small part, then repeat on smaller input [OK]

Common Mistakes:

• Confusing recursion with loops
• Ignoring the base case
• Trying to process all elements at once

2. Which of the following is the correct base case for a recursive function that processes a string character by character?

easy

A. if (str[0] == '\0') return;

B. if (str == null) return;

C. if (str.Length > 0) return;

D. if (str.Length == 0) return;

Solution

1. Step 1: Identify base case for string recursion

   The base case should stop recursion when the string is empty (length zero).

2. Step 2: Check options

   if (str.Length == 0) return; correctly checks if string length is zero to stop recursion.

3. Final Answer:

   if (str.Length == 0) return; -> Option D

4. Quick Check:

   Base case = empty string check [OK]

Hint: Base case usually checks for empty string or array length zero [OK]

Common Mistakes:

• Checking for null instead of empty string
• Using wrong condition like length > 0
• Checking for null character '\0' in C# strings

3. What will be the output of this C# recursive function call?

static int SumArray(int[] arr, int index) {
  if (index == arr.Length) return 0;
  return arr[index] + SumArray(arr, index + 1);
}

int[] nums = {1, 2, 3};
Console.WriteLine(SumArray(nums, 0));

medium

A. 6

B. 0

C. 3

D. StackOverflowException

Solution

1. Step 1: Trace recursive calls

   SumArray called with index 0 returns arr[0] + SumArray(1) = 1 + ... SumArray(1) returns 2 + SumArray(2) SumArray(2) returns 3 + SumArray(3) SumArray(3) returns 0 (base case)

2. Step 2: Calculate total sum

   Sum = 1 + 2 + 3 + 0 = 6

3. Final Answer:

   6 -> Option A

4. Quick Check:

   Sum of [1,2,3] = 6 [OK]

Hint: Add current element + recursive sum of rest [OK]

Common Mistakes:

• Forgetting base case leads to infinite recursion
• Returning wrong index or value
• Confusing index with length

4. Identify the error in this recursive function that reverses a string:

static string Reverse(string s) {
  if (s.Length == 0) return s;
  return Reverse(s.Substring(1)) + s[0];
}

// Usage: Console.WriteLine(Reverse(null));

medium

A. It causes a NullReferenceException when input is null

B. It never reaches the base case

C. It returns the original string without reversing

D. It causes infinite recursion

Solution

1. Step 1: Analyze input null handling

   The function calls s.Length but if s is null, this causes NullReferenceException.

2. Step 2: Check base case and usage

   Base case is correct for empty string but does not handle null input, causing runtime error.

3. Final Answer:

   It causes a NullReferenceException when input is null -> Option A

4. Quick Check:

   Null input causes exception [OK]

Hint: Always check for null before accessing string properties [OK]

Common Mistakes:

• Not checking for null input
• Assuming empty string and null are same
• Ignoring exceptions from null dereference

5. Given a string, write a recursive function to check if it is a palindrome (reads same forward and backward). Which of these approaches is correct?

hard

A. Use recursion to sort the string and compare with original

B. Check if first and last characters are equal, then recurse on substring excluding them

C. Count characters recursively and check if count is even

D. Reverse the string using recursion and compare with original

Solution

1. Step 1: Understand palindrome check logic

   Palindrome means first and last characters match, and inner substring is also palindrome.

2. Step 2: Apply recursion

   Recursively check first and last chars, then call function on substring excluding them until base case (empty or single char).

3. Final Answer:

   Check if first and last characters are equal, then recurse on substring excluding them -> Option B

4. Quick Check:

   Palindrome check = compare ends + recurse [OK]

Hint: Compare ends, then recurse inside substring [OK]

Common Mistakes:

• Reversing string is less efficient
• Counting characters doesn't prove palindrome
• Sorting changes string order