Recursion on Arrays and Strings in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to return the length of the string using recursion.

DSA Typescript

function recursiveLength(str: string): number {
  if (str === "") {
    return 0;
  }
  return 1 + recursiveLength(str[1]);
}

Drag options to blanks, or click blank then click option'

A.slice(1)

B.charAt(1)

C.substring(0, 1)

D.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using str.length instead of slicing the string.

Using substring(0,1) which returns the first character, not the rest.

We use str.slice(1) to get the string without the first character and call recursion on it.

2fill in blank

medium

Complete the code to check if an array contains a target value using recursion.

DSA Typescript

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

Drag options to blanks, or click blank then click option'

A.index

B[index]

C(index)

D{index}

Attempts:

3 left

💡 Hint

Common Mistakes

Using dot notation which is invalid for array indexing.

Using parentheses which is function call syntax.

We access the element at the current index using arr[index] to compare with the target.

3fill in blank

hard

Fix the error in the recursive function that reverses a string.

DSA Typescript

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

Drag options to blanks, or click blank then click option'

A.slice(1)

B.charAt(1)

C.substring(1)

D.slice(0, 1)

Attempts:

3 left

💡 Hint

Common Mistakes

Using substring(1) works but slice(1) is preferred for clarity.

Using slice(0,1) returns the first character, not the rest.

We use str.slice(1) to get the string without the first character for recursion.

4fill in blank

hard

Fill both blanks to create a recursive function that sums all elements in an array.

DSA Typescript

function sumArray(arr: number[], index: number = 0): number {
  if (index [1] arr.length) {
    return 0;
  }
  return arr[2] + sumArray(arr, index + 1);
}

Drag options to blanks, or click blank then click option'

A<

B>

C[index]

D.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in the base case condition.

Using dot notation instead of square brackets for array access.

We check if index is less than array length and access element at arr[index].

5fill in blank

hard

Fill all three blanks to create a recursive function that counts how many times a character appears in a string.

DSA Typescript

function countChar(str: string, char: string, index: number = 0): number {
  if (index [1] str.length) {
    return 0;
  }
  const count = (str.charAt(index) === char) ? [2] : 0;
  return count + countChar(str, char, index [3] 1);
}

Drag options to blanks, or click blank then click option'

A<

B1

C+

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using '>' instead of '<' in the base case.

Adding 0 instead of 1 when characters match.

Using '-' instead of '+' to increment index.

We check if index is less than string length, add 1 if characters match, and increment index by 1.