String Reversal Approaches in DSA Typescript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of String Reversal Using Array Methods

What is the output of the following TypeScript code that reverses a string using array methods?

DSA Typescript

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

A "hello"

B "olleh"

C "ohlle"

D "helol"

💡 Hint

Think about what split, reverse, and join do to the string.

The code splits the string into characters, reverses the array, then joins it back into a string. So "hello" becomes "olleh".

❓ Predict Output

intermediate

Output of Manual String Reversal Using Loop

What is the output of this TypeScript code that reverses a string manually using a loop?

DSA Typescript

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

A "edcba"

B "abcde"

C "edcab"

D "abc"

💡 Hint

The loop starts from the end of the string and adds characters to reversed.

The loop goes from the last character to the first, building the reversed string character by character.

🔧 Debug

advanced

Identify the Error in Recursive String Reversal

What error does this TypeScript recursive string reversal function produce when called with "test"?

DSA Typescript

function reverseString(s: string): string {
  if (s.length === 0) return s;
  return reverseString(s.substring(1)) + s[0];
}
console.log(reverseString("test"));

A "tset"

B Stack overflow error

C TypeError: s.substring is not a function

D "test"

💡 Hint

Check the base case and how substring is used.

The function correctly reverses the string recursively by calling itself on the substring from index 1 and appending the first character at the end. It returns "tset" for "test".

❓ Predict Output

advanced

Output of String Reversal Using Spread Operator and Reduce

What is the output of this TypeScript code that reverses a string using the spread operator and reduce?

DSA Typescript

const str = "world";
const reversed = [...str].reduce((acc, char) => char + acc, '');
console.log(reversed);

A "world"

B "wrold"

C "dlrow"

D "dlorw"

💡 Hint

Reduce accumulates characters by adding current char before accumulator.

The reduce function prepends each character to the accumulator, effectively reversing the string.

🧠 Conceptual

expert

Time Complexity of Different String Reversal Methods

Which option correctly describes the time complexity of reversing a string of length n using a manual loop versus using built-in methods like split, reverse, and join?

A Both manual loop and built-in methods have O(n) time complexity.

B Both manual loop and built-in methods have O(1) time complexity.

C Manual loop is O(n); built-in methods are O(n^2) because of array operations.

D Manual loop is O(n^2) due to string concatenation; built-in methods are O(n).

💡 Hint

Consider how string concatenation inside a loop affects performance.

Manual reversal using string concatenation inside a loop can be O(n^2) because strings are immutable and concatenation creates new strings each time. Built-in methods like split, reverse, and join operate on arrays and are optimized to run in O(n).

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