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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the function call itself recursively.

DSA C

void countdown(int n) {
    if (n == 0) {
        printf("Blastoff!\n");
        return;
    }
    printf("%d\n", n);
    countdown([1]);
}

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An + 1

Bn

Cn - 1

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Calling countdown with n + 1 causes infinite recursion.

Calling countdown with n does not reduce the problem size.

The function calls itself with n - 1 to count down until it reaches zero.

2fill in blank

medium

Complete the code to calculate factorial of n recursively.

DSA C

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

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An - 1

Bn + 1

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using 1 or 0 directly does not compute factorial correctly.

Factorial of n is n times factorial of n-1, moving towards base case 0.

3fill in blank

hard

Fix the error in the recursive Fibonacci function call.

DSA C

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

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An + 1

Bn - 1

Cn

D2

Attempts:

3 left

💡 Hint

Common Mistakes

Using n + 1 causes infinite recursion.

Using n or 2 as argument breaks the Fibonacci logic.

Fibonacci is sum of fibonacci(n-1) and fibonacci(n-2).

4fill in blank

hard

Fill both blanks to complete the recursive sum of array elements.

DSA C

int sumArray(int arr[], int size) {
    if (size == 0) {
        return 0;
    }
    return arr[[1]] + sumArray(arr, [2]);
}

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Asize - 1

Bsize + 1

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using size or size + 1 causes out-of-bounds errors.

Not reducing size leads to infinite recursion.

We add the last element arr[size-1] and sum the rest with size-1.

5fill in blank

hard

Fill all three blanks to complete the recursive power function.

DSA C

int power(int base, int exp) {
    if (exp == 0) {
        return [1];
    }
    return base * power(base, [2]);
}

// Usage: int result = power([3], 3);

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A1

Bexp - 1

C2

Dbase

Attempts:

3 left

💡 Hint

Common Mistakes

Returning base instead of 1 in base case.

Not reducing exponent causes infinite recursion.

Using wrong base value in usage example.

Base case returns 1 for exponent 0; recursive call reduces exponent by 1; usage example uses base.