Recursion Concept and Call Stack Visualization in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the recursive method call itself with a smaller number.

DSA Java

public int factorial(int n) {
    if (n == 0) {
        return 1;
    } else {
        return n * factorial([1]);
    }
}

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

An

Bn + 1

Cn - 1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n causes no change and infinite recursion.

The recursive call must reduce the problem size by calling factorial with n - 1.

2fill in blank

medium

Complete the code to correctly check the base case for the recursive Fibonacci method.

DSA Java

public int fibonacci(int n) {
    if (n <= [1]) {
        return n;
    } else {
        return fibonacci(n - 1) + fibonacci(n - 2);
    }
}

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

A2

B1

C0

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as base case alone misses the 1 case.

Using n as base case causes infinite recursion.

The base cases for Fibonacci are when n <= 1, returning n (0 or 1).

3fill in blank

hard

Fix the error in the recursive sum method by completing the base case condition.

DSA Java

public int sum(int[] arr, int index) {
    if (index [1] arr.length) {
        return 0;
    } else {
        return arr[index] + sum(arr, index + 1);
    }
}

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

A>=

B<=

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using > causes index out of bounds error.

Using < causes the method to always return 0.

The base case is when index is greater than or equal to array length, stopping recursion.

4fill in blank

hard

Fill both blanks to complete the recursive method that prints numbers from n down to 1.

DSA Java

public void printNumbers(int n) {
    if (n [1] 0) {
        return;
    }
    System.out.println(n);
    printNumbers(n [2] 1);
}

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

A<=

B-

C>

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using + instead of - causes infinite recursion.

Using <= in the if condition causes wrong stopping point.

The method stops when n is not greater than 0, and calls itself with n - 1 to count down.

5fill in blank

hard

Fill all three blanks to complete the recursive method that reverses a string.

DSA Java

public String reverse(String str) {
    if (str.length() == 0) {
        return "";
    }
    return str.charAt([1]) + reverse(str.[2]([3], str.length() - 1));
}

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

Astr.length() - 1

Bsubstring

C0

Dlength

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as index gets first character, not last.

Using length as substring argument causes error.

The method takes the last character (index length-1) and adds it to the reverse of the substring from 0 to length-1.

Practice

1. What happens when a recursive function calls itself without a stopping condition?

easy

A. It converts into a loop automatically.

B. It runs only once and stops automatically.

C. It causes infinite recursion leading to a stack overflow error.

D. It returns null immediately.

Solution

1. Step 1: Understand recursion without base case

   A recursive function without a stopping condition keeps calling itself endlessly.

2. Step 2: Effect on call stack

   Each call adds a new frame to the call stack until memory runs out, causing a stack overflow error.

3. Final Answer:

   It causes infinite recursion leading to a stack overflow error. -> Option C

4. Quick Check:

   Missing base case = stack overflow [OK]

Hint: Always check for a base case to stop recursion [OK]

Common Mistakes:

• Assuming recursion stops automatically
• Thinking recursion converts to loop
• Believing it returns null without base case

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

easy

A. public void recurse() { recurse(); }

B. public void recurse() { return recurse(); }

C. public void recurse() { if(true recurse(); }

D. public void recurse() { recurse; }

Solution

1. Step 1: Check method call syntax

   public void recurse() { recurse(); } correctly calls the method recurse() inside itself using parentheses.

2. Step 2: Identify syntax errors in other options

   public void recurse() { return recurse(); } tries to return void method call, invalid. public void recurse() { if(true recurse(); } missing parenthesis after if condition, invalid syntax. public void recurse() { recurse; } misses parentheses, invalid call.

3. Final Answer:

   public void recurse() { recurse(); } -> Option A

4. Quick Check:

   Method call needs parentheses [OK]

Hint: Recursive calls must use parentheses like normal method calls [OK]

Common Mistakes:

• Omitting parentheses in recursive call
• Returning void method call
• Using invalid syntax inside method

3. What will be the output of the following Java code?

public class Test {
  public static void printNumbers(int n) {
    if (n == 0) return;
    System.out.print(n + " ");
    printNumbers(n - 1);
  }
  public static void main(String[] args) {
    printNumbers(3);
  }
}

medium

A. 1 2 3

B. 0 1 2 3

C. 3 2 1 0

D. 3 2 1

Solution

1. Step 1: Trace recursive calls and prints

   printNumbers(3) prints 3, then calls printNumbers(2), prints 2, calls printNumbers(1), prints 1, then calls printNumbers(0) which returns.

2. Step 2: Output order

   Numbers print before recursive call, so output is "3 2 1 " with no zero printed.

3. Final Answer:

   3 2 1 -> Option D

4. Quick Check:

   Print before recursion = descending order [OK]

Hint: Print before recursion call prints descending sequence [OK]

Common Mistakes:

• Expecting ascending order output
• Including zero in output
• Confusing print order with recursion

4. Identify the error in the following recursive Java method and select the fix:

public static int factorial(int n) {
  if (n == 1) return 1;
  else return n * factorial(n);
}

medium

A. Add print statement before recursive call

B. Change recursive call to factorial(n - 1)

C. Change base case to if (n == 0) return 0

D. Change return type to void

Solution

1. Step 1: Analyze recursive call parameter

   The method calls factorial(n) again without reducing n, causing infinite recursion.

2. Step 2: Correct recursive call

   It should call factorial(n - 1) to reduce n and reach base case.

3. Final Answer:

   Change recursive call to factorial(n - 1) -> Option B

4. Quick Check:

   Recursive call must reduce parameter [OK]

Hint: Recursive calls must move toward base case by changing parameters [OK]

Common Mistakes:

• Calling factorial with same argument causing infinite recursion
• Wrong base case value
• Changing return type unnecessarily

5. Consider the following recursive method to compute the sum of digits of a positive integer:

public static int sumDigits(int n) {
  if (n == 0) return 0;
  return (n % 10) + sumDigits(n / 10);
}

What is the output of sumDigits(1234) and explain how the call stack evolves?

hard

A. 10; Calls: sumDigits(1234) -> sumDigits(123) -> sumDigits(12) -> sumDigits(1) -> sumDigits(0)

B. 1234; Calls: sumDigits(1234) only once

C. 0; Calls: sumDigits(0) only

D. 15; Calls: sumDigits(1234) -> sumDigits(123) -> sumDigits(12) -> sumDigits(1) -> sumDigits(0)

Solution

1. Step 1: Trace recursive calls and sums

   sumDigits(1234) = 4 + sumDigits(123), sumDigits(123) = 3 + sumDigits(12), sumDigits(12) = 2 + sumDigits(1), sumDigits(1) = 1 + sumDigits(0), sumDigits(0) = 0.

2. Step 2: Calculate total sum and call stack order

   Total sum = 4+3+2+1+0 = 10. The call stack grows with calls: 1234 -> 123 -> 12 -> 1 -> 0, then unwinds adding digits.

3. Final Answer:

   10; Calls: sumDigits(1234) -> sumDigits(123) -> sumDigits(12) -> sumDigits(1) -> sumDigits(0) -> Option A

4. Quick Check:

   Sum digits = 10, call stack grows with decreasing n [OK]

Hint: Sum digits by adding last digit and recurse on rest [OK]

Common Mistakes:

• Adding digits incorrectly
• Misunderstanding call stack order
• Confusing sum with original number