String Reversal Approaches in DSA Typescript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse a string using built-in methods.

DSA Typescript

const reversed = str.split('')[1].join('');
console.log(reversed);

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

Asort()

Breverse()

Cmap()

Dfilter()

Attempts:

3 left

💡 Hint

Common Mistakes

Using sort() instead of reverse()

Trying to reverse string directly without splitting

The reverse() method reverses the array elements, which helps reverse the string after splitting it.

2fill in blank

medium

Complete the code to reverse a string using a for loop.

DSA Typescript

let reversed = '';
for (let i = str.length - 1; i [1] 0; i--) {
  reversed += str[i];
}
console.log(reversed);

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

A<=

B>

C>=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using < instead of >=

Looping forwards instead of backwards

The loop starts from the last index and continues while i is greater than or equal to 0 to include the first character.

3fill in blank

hard

Fix the error in swapping characters to reverse a string in place using an array.

DSA Typescript

const arr = str.split('');
for (let i = 0; i < arr.length / 2; i++) {
  const temp = arr[i];
  arr[i] = arr[arr.length - 1 - [1]];
  arr[arr.length - 1 - i] = temp;
}
const reversed = arr.join('');
console.log(reversed);

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

Ai

Btemp

Carr.length

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using temp instead of i

Using arr.length directly causing out of bounds

The index to swap with is arr.length - 1 - i, so i must be used in the expression.

4fill in blank

hard

Fill both blanks to create a function that reverses a string using recursion.

DSA Typescript

function reverse(str: string): string {
  if (str.length [1] 0) return '';
  return str.charAt(str.length - 1) + reverse(str.[2](0, -1));
}

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

A===

B>

Cslice

Dsubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using > instead of === in base case

Using substring instead of slice with negative index

The base case checks if the string length is zero using ===. The recursive call uses slice(0, -1) to remove the last character.

5fill in blank

hard

Fill all three blanks to create a one-liner that reverses a string using reduce.

DSA Typescript

const reversed = str.split('')[1]((acc, char) => char [2] acc, [3]);
console.log(reversed);

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

AreduceRight

Breduce

C+

D''

Attempts:

3 left

💡 Hint

Common Mistakes

Using reduceRight instead of reduce

Using subtraction operator instead of addition

Missing initial value for reduce

The reduce method accumulates characters in reverse order by adding the current character before the accumulator. The initial value is an empty string.

Practice

1. What does the split('') method do when applied to a string in TypeScript?

easy

A. It reverses the string directly.

B. It converts the string into an array of individual characters.

C. It joins the string elements into one string.

D. It removes all spaces from the string.

Solution

1. Step 1: Understand the split method

   The split('') method breaks a string into an array of characters by splitting at every empty string between characters.

2. Step 2: Effect on string

   For example, "hello".split('') becomes ["h", "e", "l", "l", "o"].

3. Final Answer:

   It converts the string into an array of individual characters. -> Option B

4. Quick Check:

   split('') = array of chars [OK]

Hint: split('') breaks string into chars array [OK]

Common Mistakes:

• Thinking split('') reverses the string
• Confusing split('') with join()
• Assuming split('') removes spaces

2. Which of the following is the correct syntax to reverse a string str using built-in methods in TypeScript?

easy

A. str.split('').reverse().join('')

B. str.reverse().join('')

C. reverse(str).split('')

D. str.join('').reverse()

Solution

1. Step 1: Split string into array

   Use str.split('') to convert string into array of characters.

2. Step 2: Reverse and join

   Use reverse() on array, then join('') to form reversed string.

3. Final Answer:

   str.split('').reverse().join('') -> Option A

4. Quick Check:

   split + reverse + join = reversed string [OK]

Hint: Split, reverse array, then join to reverse string [OK]

Common Mistakes:

• Calling reverse() directly on string
• Using join() before reverse()
• Using reverse() as a standalone function

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

const str = "abcde";
const reversed = str.split('').reverse().join('');
console.log(reversed);

medium

A. "edcbe"

B. "abcde"

C. ""

D. "edcba"

Solution

1. Step 1: Split string into array

   "abcde".split('') becomes ["a", "b", "c", "d", "e"].

2. Step 2: Reverse and join array

   Reversing gives ["e", "d", "c", "b", "a"], joining results in "edcba".

3. Final Answer:

   "edcba" -> Option D

4. Quick Check:

   split + reverse + join = "edcba" [OK]

Hint: Split, reverse, join outputs reversed string [OK]

Common Mistakes:

• Expecting original string output
• Mixing up reversed characters
• Forgetting to join after reverse

4. Identify the error in this TypeScript function intended to reverse a string:

function reverseString(s: string): string {
  return s.reverse();
}

medium

A. Strings do not have a reverse() method directly.

B. The function should return void, not string.

C. The parameter should be an array, not a string.

D. The function is missing a return statement.

Solution

1. Step 1: Check string methods

   Strings in TypeScript do not have a reverse() method directly; only arrays do.

2. Step 2: Correct approach

   Convert string to array with split(''), reverse array, then join back.

3. Final Answer:

   Strings do not have a reverse() method directly. -> Option A

4. Quick Check:

   reverse() works on arrays, not strings [OK]

Hint: reverse() is for arrays, not strings [OK]

Common Mistakes:

• Trying to call reverse() on string directly
• Ignoring need to split string first
• Assuming reverse() returns void

5. Given the function below, what will be the output when calling reverseString('12345')?

function reverseString(s: string): string {
  let reversed = '';
  for (let i = s.length - 1; i >= 0; i--) {
    reversed += s[i];
  }
  return reversed;
}

hard

A. "5432"

B. "12345"

C. "54321"

D. ""

Solution

1. Step 1: Loop from end to start

   The loop starts at index 4 (last char '5') and moves backward to 0.

2. Step 2: Build reversed string

   Characters are appended in reverse order: '5', '4', '3', '2', '1'.

3. Final Answer:

   "54321" -> Option C

4. Quick Check:

   Loop backward concatenates reversed string [OK]

Hint: Loop backward to build reversed string [OK]

Common Mistakes:

• Looping forward instead of backward
• Off-by-one errors in loop index
• Not initializing reversed string properly