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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Simple Recursive Sum Function

What is the output of the following TypeScript code that uses recursion to sum numbers from n down to 1?

DSA Typescript

function recursiveSum(n: number): number {
  if (n <= 0) return 0;
  return n + recursiveSum(n - 1);
}

console.log(recursiveSum(4));

A 0

B 4

C 10

D Undefined (runtime error)

💡 Hint

Think about how the function adds n to the sum of all numbers below it.

The function adds n to the sum of numbers from n-1 down to 1 recursively. For n=4, it calculates 4 + 3 + 2 + 1 = 10.

❓ Predict Output

intermediate

Output of Recursive Factorial Function

What will be printed when this recursive factorial function is called with 3?

DSA Typescript

function factorial(n: number): number {
  if (n === 0) return 1;
  return n * factorial(n - 1);
}

console.log(factorial(3));

A 9

B 6

C 0

D Error: Maximum call stack size exceeded

💡 Hint

Factorial multiplies n by factorial of n-1 until it reaches 0.

factorial(3) = 3 * factorial(2) = 3 * 2 * factorial(1) = 3 * 2 * 1 * factorial(0) = 3 * 2 * 1 * 1 = 6.

🔧 Debug

advanced

Identify the Error in Recursive Fibonacci Function

What is the output of this TypeScript code when calling fibonacci(5)?

DSA Typescript

function fibonacci(n: number): number {
  if (n === 0) return 0;
  if (n === 1) return 1;
  return fibonacci(n - 1) + fibonacci(n - 2);
}

console.log(fibonacci(5));

A 5

B 8

C Stack overflow error

D TypeError: fibonacci is not a function

💡 Hint

Trace fibonacci(5) calls and sum results.

fibonacci(5) = fibonacci(4) + fibonacci(3) = 3 + 2 = 5. The code runs correctly without errors.

❓ Predict Output

advanced

Output of Recursive Function with Multiple Calls

What is the output of this recursive function that prints numbers during the call and return phases?

DSA Typescript

function printNumbers(n: number): void {
  if (n <= 0) return;
  console.log('Call:', n);
  printNumbers(n - 1);
  console.log('Return:', n);
}

printNumbers(3);

A "Call: 3" "Call: 2" "Call: 1" "Return: 1" "Return: 2" "Return: 3"

B "Return: 1" "Return: 2" "Return: 3" "Call: 3" "Call: 2" "Call: 1"

C "Call: 1" "Call: 2" "Call: 3" "Return: 3" "Return: 2" "Return: 1"

D "Call: 3" "Return: 3" "Call: 2" "Return: 2" "Call: 1" "Return: 1"

💡 Hint

Notice the order of console.log before and after the recursive call.

The function prints 'Call:' before going deeper and 'Return:' after returning from recursion, so calls print down then returns print up.

🧠 Conceptual

expert

Maximum Call Stack Depth in Recursive Function

Consider this recursive function without a base case. What will happen when calling infiniteRecursion(1)?

DSA Typescript

function infiniteRecursion(n: number): number {
  return infiniteRecursion(n + 1) + 1;
}

infiniteRecursion(1);

A Returns Infinity

B Returns 1

C Runs forever without error

D Throws RangeError: Maximum call stack size exceeded

💡 Hint

Think about what happens when recursion never stops.

Without a base case, the function calls itself endlessly until the system runs out of stack space, causing a RangeError.