Recursion on Arrays and Strings in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Sum of Array Elements

What is the output of this TypeScript code that sums elements of an array recursively?

DSA Typescript

function recursiveSum(arr: number[], index: number = 0): number {
  if (index === arr.length) return 0;
  return arr[index] + recursiveSum(arr, index + 1);
}

const result = recursiveSum([1, 2, 3, 4]);
console.log(result);

A TypeError

B 10

C 0

D 24

💡 Hint

Think about how the function adds each element starting from index 0 until the end.

The function adds each element starting from index 0 recursively until it reaches the end, summing 1+2+3+4 = 10.

❓ Predict Output

intermediate

Output of Recursive String Reverse

What is the output of this TypeScript code that reverses a string recursively?

DSA Typescript

function reverseString(str: string): string {
  if (str === "") return "";
  return reverseString(str.substr(1)) + str.charAt(0);
}

const reversed = reverseString("code");
console.log(reversed);

A "

B code

C edoc

D RangeError

💡 Hint

The function takes the first character and puts it at the end after reversing the rest.

The function reverses the string by recursively calling itself on the substring excluding the first character and appending the first character at the end.

🔧 Debug

advanced

Identify the Error in Recursive Array Search

What error does this TypeScript code produce when searching for a value recursively in an array?

DSA Typescript

function recursiveSearch(arr: number[], target: number, index: number = 0): boolean {
  if (index >= arr.length) return false;
  if (arr[index] === target) return true;
  return recursiveSearch(arr, target, index + 1);
}

console.log(recursiveSearch([5, 3, 7], 7));

A No error, outputs true

B TypeError

C undefined is not a number

D StackOverflowError

💡 Hint

Check the base case condition for index exceeding array length.

The base case was originally 'index > arr.length' which allowed index to equal arr.length causing arr[index] to be undefined and comparing undefined to target causes a TypeError. Changing it to 'index >= arr.length' fixes the error.

❓ Predict Output

advanced

Output of Recursive Flattening of Nested Arrays

What is the output of this TypeScript code that recursively flattens a nested array?

DSA Typescript

function flatten(arr: any[]): any[] {
  if (arr.length === 0) return [];
  const [first, ...rest] = arr;
  if (Array.isArray(first)) {
    return [...flatten(first), ...flatten(rest)];
  } else {
    return [first, ...flatten(rest)];
  }
}

const nested = [1, [2, [3, 4], 5], 6];
console.log(flatten(nested));

A [1, 2, 3, 4, 5, 6]

B [1, [2, [3, 4], 5], 6]

C [[1, 2], 3, 4, 5, 6]

D TypeError

💡 Hint

The function checks if the first element is an array and flattens it recursively.

The function recursively flattens nested arrays by concatenating flattened first element and flattened rest of the array.

🧠 Conceptual

expert

Maximum Call Stack Depth in Recursive Palindrome Check

Consider this recursive TypeScript function that checks if a string is a palindrome. What is the maximum depth of the call stack for a string of length n?

DSA Typescript

function isPalindrome(str: string, left: number = 0, right: number = str.length - 1): boolean {
  if (left >= right) return true;
  if (str[left] !== str[right]) return false;
  return isPalindrome(str, left + 1, right - 1);
}

A n - 1

B n

C Math.ceil(n / 2)

D Math.floor(n / 2)

💡 Hint

Each recursive call moves inward from both ends by one character.

The function calls itself reducing the problem size by two characters each time, so maximum depth is floor of n/2.