Recursion vs Iteration When Each Wins in DSA Typescript - Compare & Choose

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Fibonacci Function

What is the output of this TypeScript code that calculates Fibonacci numbers recursively?

DSA Typescript

function fib(n: number): number {
  if (n <= 1) return n;
  return fib(n - 1) + fib(n - 2);
}
console.log(fib(5));

A 8

B 3

C 13

D 5

💡 Hint

Trace the calls for fib(5): fib(4) + fib(3).

The Fibonacci sequence starts 0,1,1,2,3,5... fib(5) returns 5.

🧠 Conceptual

intermediate

When is Iteration Better Than Recursion?

Which scenario is best suited for iteration instead of recursion?

A When the problem involves simple loops with large input sizes to avoid stack overflow

B When the problem requires exploring all branches of a tree

C When the problem has a small fixed depth and no repeated calculations

D When the problem naturally fits a divide-and-conquer approach

💡 Hint

Think about stack limits and performance with large inputs.

Iteration avoids stack overflow and is efficient for simple loops with large inputs.

🔧 Debug

advanced

Identify the Error in Recursive Factorial

What error will this TypeScript code produce when calculating factorial?

DSA Typescript

function factorial(n: number): number {
  if (n === 0) return 1;
  return n * factorial(n);
}
console.log(factorial(4));

A Returns 1 always

B Returns 24 correctly

C Stack overflow error due to infinite recursion

D Syntax error due to missing return type

💡 Hint

Check the recursive call argument.

The function calls factorial(n) instead of factorial(n-1), causing infinite recursion.

❓ Predict Output

advanced

Output of Iterative vs Recursive Sum

What is the output of this TypeScript code that sums numbers from 1 to 5 using iteration and recursion?

DSA Typescript

function sumIterative(n: number): number {
  let total = 0;
  for (let i = 1; i <= n; i++) {
    total += i;
  }
  return total;
}

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

console.log(sumIterative(5));
console.log(sumRecursive(5));

A 10 and 15

B 15 and 15

C 15 and 10

D 10 and 10

💡 Hint

Sum of numbers 1 to 5 is 15.

Both functions correctly calculate the sum 1+2+3+4+5 = 15.

🧠 Conceptual

expert

Why Use Recursion Over Iteration?

Which reason best explains when recursion is preferred over iteration?

A When the problem is naturally hierarchical or tree-structured and easier to express recursively

B When performance and memory usage must be minimized at all costs

C When the problem involves simple counting loops with no nested calls

D When the language does not support loops

💡 Hint

Think about problem structure and clarity.

Recursion is preferred for problems like tree traversal where the structure fits recursive calls.

Practice

1. Which scenario is best suited for using recursion instead of iteration?

easy

A. When the problem requires simple repeated loops

B. When you want to avoid function call overhead

C. When the problem can be divided into similar smaller problems

D. When memory usage must be minimal

Solution

1. Step 1: Understand recursion vs iteration use cases

   Recursion works well when a problem can be broken down into smaller, similar subproblems, like tree traversals or factorial calculation. Iteration is better for simple repeated tasks without nested subproblems, and it avoids function call overhead.

2. Final Answer:

   When the problem can be divided into similar smaller problems -> Option C

3. Quick Check:

   Recursion = divide and conquer [OK]

Hint: Recursion fits problems breaking into smaller similar parts [OK]

Common Mistakes:

• Thinking iteration is always faster
• Ignoring function call overhead in recursion
• Confusing recursion with simple loops

2. Which of the following is the correct syntax to write a recursive function in TypeScript?

easy

A. function recurse(n: number): number { if (n <= 1) return 1; return n * recurse(n - 1); }

B. function recurse(n: number): number { while (n > 1) { n--; } return n; }

C. function recurse(n: number): number { for (let i = 0; i < n; i++) { return i; } }

D. function recurse(n: number): number { if (n == 0) return 0; else return n + 1; }

Solution

1. Step 1: Identify recursive call and base case

   function recurse(n: number): number { if (n <= 1) return 1; return n * recurse(n - 1); }: function recurse(n: number): number { if (n <= 1) return 1; return n * recurse(n - 1); } calls itself inside the function with a smaller argument (n-1) and has a base case (n <= 1) to stop recursion.

2. Final Answer:

   function recurse(n: number): number { if (n <= 1) return 1; return n * recurse(n - 1); } -> Option A

3. Quick Check:

   Recursive function must call itself with smaller input [OK]

Hint: Look for function calling itself with smaller input [OK]

Common Mistakes:

• Missing base case causing infinite recursion
• Using loops instead of recursive calls
• Returning inside a loop without recursion

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

function factorial(n: number): number {
  if (n <= 1) return 1;
  return n * factorial(n - 1);
}
console.log(factorial(4));

medium

A. 10

B. 24

C. 16

D. Error

Solution

1. Step 1: Trace recursive calls for factorial(4)

   factorial(4) calls factorial(3), which calls factorial(2), which calls factorial(1)=1. Then: factorial(2)=2*1=2, factorial(3)=3*2=6, factorial(4)=4*6=24. Output: 24.

2. Final Answer:

   24 -> Option B

3. Quick Check:

   4! = 24 [OK]

Hint: Factorial n! = n * (n-1)! [OK]

Common Mistakes:

• Confusing factorial with sum
• Forgetting base case returns 1
• Miscounting recursive calls

4. Identify the error in this recursive function and choose the fix:

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

medium

A. Add a base case to stop recursion when n <= 0

B. Change return to console.log to print values

C. Replace recursion with a for loop

D. Remove the parameter n

Solution

1. Step 1: Identify and fix the missing base case

   The function calls itself without a stopping condition, causing infinite recursion and stack overflow. Add if (n <= 0) return 0; to stop recursion when n <= 0.

2. Final Answer:

   Add a base case to stop recursion when n <= 0 -> Option A

3. Quick Check:

   Base case prevents infinite recursion [OK]

Hint: Always add base case to recursive functions [OK]

Common Mistakes:

• Forgetting base case causes infinite recursion
• Trying to fix by removing parameters
• Confusing printing with returning

5. You need to process a large dataset with a depth of 10,000 levels. Which approach is better and why?

Options:
A) Use recursion because it is easier to write.
B) Use recursion with no base case.
C) Use iteration to avoid stack overflow errors.
D) Use iteration but with no loop control.

hard

A. Use recursion because it is easier to write.

B. Use recursion with no base case.

C. Use iteration but with no loop control.

D. Use iteration to avoid stack overflow errors.

Solution

1. Step 1: Analyze stack limits for deep recursion

   Recursion uses the call stack, which has limited size. Depth of 10,000 risks stack overflow. Iteration uses loops without adding to call stack, safely handling large depths.

2. Final Answer:

   Use iteration to avoid stack overflow errors. -> Option D

3. Quick Check:

   Deep tasks need iteration to prevent stack overflow [OK]

Hint: Deep recursion risks stack overflow; prefer iteration [OK]

Common Mistakes:

• Assuming recursion always better for readability
• Ignoring stack size limits
• Using recursion without base case