String Reversal Approaches in DSA Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the simplest way to reverse a string in TypeScript using built-in methods?

Split the string into an array of characters, reverse the array, then join it back into a string.

Example: str.split('').reverse().join('')

intermediate

How does the two-pointer approach work for reversing a string?

Use two pointers: one at the start and one at the end of the string (or array). Swap characters at these pointers and move them towards the center until they meet.

beginner

Why can't strings be reversed by swapping characters directly in TypeScript?

Strings are immutable in TypeScript, so you cannot change characters directly. You must convert the string to an array, modify it, then convert back.

advanced

What is a recursive approach to reverse a string?

Recursively call the function with the substring excluding the first character, then append the first character at the end until the string is empty.

intermediate

What is the time complexity of reversing a string using built-in methods in TypeScript?

The time complexity is O(n), where n is the length of the string, because each character is processed once during split, reverse, and join.

Which TypeScript method converts a string into an array of characters?

A split('')

B reverse()

C join('')

D slice()

The split('') method splits a string into an array of individual characters.

Why do we convert a string to an array to reverse it in TypeScript?

A Because arrays are immutable

B Because strings are immutable

C Because strings have no length

D Because arrays are faster

Strings cannot be changed directly in TypeScript, so we convert to an array to modify characters.

In the two-pointer approach, what do the pointers do?

A Skip characters in the middle

B Only move from start to end

C Only move from end to start

D Swap characters from start and end moving inward

Two pointers swap characters from the start and end, moving towards the center.

What is the base case in a recursive string reversal?

A When the string length is even

B When the string length is odd

C When the string is empty or has one character

D When the string contains spaces

The recursion stops when the string is empty or has one character, as it is already reversed.

What is the time complexity of reversing a string using the two-pointer approach?

A O(n)

B O(n^2)

C O(log n)

D O(1)

Each character is swapped once, so the time complexity is linear, O(n).

Explain three different ways to reverse a string in TypeScript.

Think about built-in methods, manual swapping, and recursive logic.

Why do we need to convert a string to an array to reverse it in TypeScript? What does immutability mean here?

Consider how strings and arrays differ in mutability.

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