Recursion Concept and Call Stack Visualization in DSA C - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Factorial Function

What is the output of the following C code when calling factorial(4)?

DSA C

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

int main() {
    int result = factorial(4);
    printf("%d", result);
    return 0;
}

A 24

B 10

C 120

D 0

💡 Hint

Recall that factorial of n is n multiplied by factorial of n-1, with factorial(0) = 1.

The factorial function multiplies numbers from n down to 1 recursively. factorial(4) = 4 * 3 * 2 * 1 = 24.

❓ Predict Output

intermediate

Output of Recursive Sum Function

What will be printed after executing this C code snippet?

DSA C

int sum(int n) {
    if (n <= 0) return 0;
    return n + sum(n - 1);
}

int main() {
    printf("%d", sum(5));
    return 0;
}

A 10

B 15

C 5

D 0

💡 Hint

Sum of first n natural numbers is n*(n+1)/2.

The function adds numbers from 5 down to 1 recursively: 5+4+3+2+1=15.

🔧 Debug

advanced

Identify the Error in Recursive Fibonacci Function

What error will occur when running this C code?

DSA C

int fibonacci(int n) {
    if (n == 0) return 0;
    if (n == 1) return 1;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

int main() {
    int result = fibonacci(-1);
    printf("%d", result);
    return 0;
}

A Compilation error

B Prints 0

C Stack overflow due to infinite recursion

D Prints 1

💡 Hint

Consider what happens when n is negative in the recursive calls.

The function does not handle negative input, so calls fibonacci(-1), fibonacci(-2), etc., causing infinite recursion and stack overflow.

🧠 Conceptual

advanced

Call Stack Depth in Recursive Function

If a recursive function calls itself with n-1 until n reaches 0, what is the maximum depth of the call stack when called with n=7?

A 1

B 7

C 6

D 8

💡 Hint

Count the initial call plus all recursive calls until base case.

The call stack includes the initial call plus one call for each decrement until 0, so total 8 calls.

🚀 Application

expert

Output of Recursive String Reverse Function

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

DSA C

#include <stdio.h>
#include <string.h>

void reverse(char *str, int start, int end) {
    if (start >= end) return;
    char temp = str[start];
    str[start] = str[end];
    str[end] = temp;
    reverse(str, start + 1, end - 1);
}

int main() {
    char s[] = "hello";
    reverse(s, 0, strlen(s) - 1);
    printf("%s", s);
    return 0;
}

A olleh

B hello

C ohlle

D error

💡 Hint

The function swaps characters from start and end moving inward.

The function swaps first and last characters recursively until the middle is reached, reversing the string.