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

Recall & Review

beginner

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

Split the string into an array of characters using split(''), reverse the array with reverse(), then join it back into a string with join('').

beginner

Explain how to reverse a string using a for loop in JavaScript.

Create an empty string, then loop from the end of the original string to the start, adding each character to the new string. This builds the reversed string step-by-step.

intermediate

What is a recursive approach to reverse a string?

The function calls itself with the substring excluding the first character, then adds the first character at the end. This repeats until the string is empty, building the reversed string on return.

intermediate

Why is using a for loop to reverse a string sometimes preferred over built-in methods?

It gives more control over the process, helps understand the logic behind reversal, and can be adapted for custom behaviors or constraints where built-in methods are not allowed.

intermediate

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

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 operations.

Which JavaScript method splits a string into an array of characters?

A reverse()

B slice()

C join('')

D split('')

The split('') method breaks the string into an array of individual characters.

What does the reverse() method do when called on an array?

A Reverses the order of elements in the array

B Removes the last element

C Sorts the array alphabetically

D Converts the array to a string

The reverse() method reverses the order of elements in the array in place.

In a recursive string reversal, what is the base case?

A When the string is empty

B When the string length is 1

C When the string contains only vowels

D When the string length is even

The base case is when the string is empty, at which point recursion stops.

Which approach to string reversal uses a loop starting from the last character to the first?

A Recursive approach

B Built-in methods approach

C For loop approach

D Map method approach

The for loop approach iterates from the last character to the first to build the reversed string.

What is the output of reversing the string 'hello' using any approach?

A hello

B olleh

C helol

D ohlle

Reversing 'hello' results in 'olleh'.

Describe three different ways to reverse a string in JavaScript.

Think about array methods, loops, and function calls.

Explain why understanding multiple string reversal approaches is useful.

Consider flexibility and learning benefits.

Practice

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

easy

A. It reverses the string characters.

B. It removes all spaces from the string.

C. It joins array elements into a string.

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

Solution

1. Step 1: Understand the split method

   The split('') method splits a string into an array by each character because the separator is an empty string.

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 D

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 in JavaScript using built-in methods?

easy

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

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

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

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

Solution

1. Step 1: Understand string and array methods

   Strings do not have a reverse() method, but arrays do. So first convert string to array using split('').

2. Step 2: Reverse array and join back

   Use reverse() on the array, then join('') to convert back to string.

3. Final Answer:

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

4. Quick Check:

   split + reverse + join = reversed string [OK]

Hint: Split string, reverse array, join back [OK]

Common Mistakes:

• Trying to call reverse() directly on string
• Using join() before reverse()
• Using incorrect method order

3. What is the output of the following code?

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

medium

A. Error: reverse is not a function

B. 'abc'

C. 'cba'

D. ['c', 'b', 'a']

Solution

1. Step 1: Split string into array

   'abc'.split('') becomes ['a', 'b', 'c'].

2. Step 2: Reverse array and join

   ['a', 'b', 'c'].reverse() becomes ['c', 'b', 'a'], then .join('') makes it 'cba'.

3. Final Answer:

   'cba' -> Option C

4. Quick Check:

   split + reverse + join = 'cba' [OK]

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

Common Mistakes:

• Expecting original string output
• Expecting array output instead of string
• Confusing method order causing errors

4. Identify the error in this string reversal code:

function reverseString(s) {
  return s.reverse().join('');
}

medium

A. Strings do not have reverse() method.

B. join() cannot be used on strings.

C. Function missing return statement.

D. split('') is missing before reverse().

Solution

1. Step 1: Check string methods

   Strings do not have reverse(), so calling s.reverse() causes error.

2. Step 2: Fix by splitting string

   We must convert string to array first using s.split('') before reversing.

3. Final Answer:

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

4. Quick Check:

   No reverse() method on strings [OK]

Hint: Strings do not have reverse() method [OK]

Common Mistakes:

• Trying to reverse string directly
• Ignoring need to split string
• Misunderstanding join() usage

5. Given the function below, what will reverseString('hello world') return?

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

hard

A. 'dlrow olleh'

B. 'hello world'

C. 'helloworld'

D. Error: Cannot read property 'length' of undefined

Solution

1. Step 1: Loop from end to start

   The for loop starts at last character index and moves backward to 0, appending each character to reversed.

2. Step 2: Build reversed string

   Characters are added in reverse order, so 'hello world' becomes 'dlrow olleh'.

3. Final Answer:

   'dlrow olleh' -> Option A

4. Quick Check:

   Loop backward appends chars reversed [OK]

Hint: Loop backward to build reversed string [OK]

Common Mistakes:

• Looping forward instead of backward
• Not appending characters correctly
• Confusing string concatenation