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

Recall & Review

beginner

What is the simplest way to reverse a string in Python using slicing?

Use slicing with a step of -1: reversed_string = original_string[::-1]. This creates a new string with characters in reverse order.

intermediate

How does the two-pointer approach reverse a string?

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

beginner

Why can't strings be reversed in-place in Python?

Strings in Python are immutable, meaning they cannot be changed after creation. To reverse, you must create a new string or convert to a list first.

beginner

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

Both approaches have a time complexity of O(n), where n is the length of the string, because each character is visited once.

intermediate

How can you reverse a string using a stack?

Push each character onto a stack, then pop characters off to build the reversed string. This uses the Last-In-First-Out property of stacks.

Which Python syntax reverses a string in one line?

A string[::-1]

B string.reverse()

C reversed(string)

D string.reverse

The slicing syntax string[::-1] returns a new string reversed. string.reverse() does not exist for strings.

Why can't you reverse a string in-place in Python?

A Python does not support reversal

B Strings are too long

C Strings are stored in reverse order

D Strings are immutable

Strings cannot be changed after creation in Python, so in-place reversal is not possible.

What data structure can help reverse a string by pushing and popping characters?

A Stack

B Queue

C Linked List

D Heap

A stack uses Last-In-First-Out order, which helps reverse the order of characters.

What is the time complexity of reversing a string of length n?

A O(n^2)

B O(1)

C O(n)

D O(log n)

Reversing requires visiting each character once, so it is O(n).

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

A Move only forward swapping characters

B Swap characters from start and end moving inward

C Skip characters at odd positions

D Only swap characters at even positions

Two pointers start at opposite ends and swap characters while moving towards the center.

Explain three different ways to reverse a string in Python and their key differences.

Think about built-in features, manual swapping, and data structures.

Describe why strings in Python cannot be reversed in-place and how you can work around this limitation.

Focus on immutability and creating new strings.

Practice

1. Which of the following methods correctly reverses a string s in Python?

easy

A. Using reversed(s) without converting to string

B. Using slicing: s[::-1]

C. Using s.reverse() method

D. Using s.append() method

Solution

1. Step 1: Understand string slicing

   The slicing syntax s[::-1] creates a new string with characters in reverse order.

2. Step 2: Check other methods

   s.reverse() and s.append() are list methods, not valid for strings. reversed(s) returns an iterator, not a string, so it needs conversion.

3. Final Answer:

   Using slicing: s[::-1] -> Option B

4. Quick Check:

   String slicing with negative step reverses string [OK]

Hint: Use s[::-1] to reverse strings quickly [OK]

Common Mistakes:

• Trying to use list methods on strings
• Forgetting to convert reversed() iterator to string
• Using append() on strings

2. Which of the following code snippets correctly reverses the string text = 'hello' and prints it?

easy

A. print(text.append('o'))

B. print(text.reverse())

C. print(text[::-2])

D. print(''.join(reversed(text)))

Solution

1. Step 1: Analyze each option

   text.reverse() is invalid for strings. text[::-2] reverses with step 2, skipping characters. text.append() is invalid for strings.

2. Step 2: Confirm correct syntax

   reversed(text) returns an iterator of characters reversed. ''.join() converts it back to string. This prints 'olleh'.

3. Final Answer:

   print(''.join(reversed(text))) -> Option D

4. Quick Check:

   reversed() + join() prints reversed string [OK]

Hint: Use ''.join(reversed(s)) to reverse strings [OK]

Common Mistakes:

• Using reverse() method on strings
• Using slicing with wrong step
• Trying to append to strings

3. What is the output of the following code?

def reverse_string(s):
    result = ''
    for char in s:
        result = char + result
    return result

print(reverse_string('abc'))

medium

A. 'cba'

B. 'abc'

C. 'bac'

D. Error

Solution

1. Step 1: Trace the loop for s='abc'

   Initially, result = ''. Iteration 1: char='a', result='a' + '' = 'a'. Iteration 2: char='b', result='b' + 'a' = 'ba'. Iteration 3: char='c', result='c' + 'ba' = 'cba'.

2. Step 2: Return the reversed string

   The function returns 'cba', which is the reverse of 'abc'.

3. Final Answer:

   'cba' -> Option A

4. Quick Check:

   Prepending chars reverses string [OK]

Hint: Add each char before result to reverse string [OK]

Common Mistakes:

• Appending char instead of prepending
• Returning original string
• Confusing order of concatenation

4. The following code is intended to reverse a string but has an error. What is the error?

def reverse_str(s):
    s.reverse()
    return s

print(reverse_str('hello'))

medium

A. Missing return statement

B. Function returns None

C. Strings do not have reverse() method

D. Syntax error in function definition

Solution

1. Step 1: Check method usage on string

   The reverse() method exists for lists, not strings. Calling it on a string causes AttributeError.

2. Step 2: Understand the error cause

   Since strings are immutable, they do not support reverse(). The code will raise an error at runtime.

3. Final Answer:

   Strings do not have reverse() method -> Option C

4. Quick Check:

   reverse() is list method, not string method [OK]

Hint: Use slicing or reversed() for strings, not reverse() [OK]

Common Mistakes:

• Trying to use reverse() on strings
• Expecting reverse() to modify string in place
• Ignoring immutability of strings

5. You want to reverse only the vowels in a string s = 'hello world' while keeping other characters in place. Which approach below correctly achieves this?

hard

A. Extract vowels, reverse them, then replace vowels in original positions

B. Reverse entire string using s[::-1]

C. Use reversed(s) and join all characters

D. Replace vowels with '*' and then reverse string

Solution

1. Step 1: Understand the problem

   We want to reverse only vowels, keeping consonants and spaces fixed in their places.

2. Step 2: Analyze each option

   Reverse entire string using s[::-1] reverses whole string, changing all characters. Use reversed(s) and join all characters reverses all characters, not just vowels. Replace vowels with '*' and then reverse string replaces vowels with '*', losing original vowels. Extract vowels, reverse them, then replace vowels in original positions extracts vowels, reverses them, then puts them back in original vowel positions, preserving other characters.

3. Final Answer:

   Extract vowels, reverse them, then replace vowels in original positions -> Option A

4. Quick Check:

   Reverse vowels only by extracting and replacing [OK]

Hint: Extract vowels, reverse, then put back in place [OK]

Common Mistakes:

• Reversing entire string instead of vowels only
• Replacing vowels with placeholders without restoring
• Ignoring character positions