String Reversal Approaches in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of slicing-based string reversal

What is the output of this Python code that reverses a string using slicing?

DSA Python

s = "hello"
reversed_s = s[::-1]
print(reversed_s)

A Error

B olleh

C hello

D h e l l o

💡 Hint

Think about how slicing with a step of -1 works on strings.

The slice s[::-1] creates a new string by taking characters from the end to the start, effectively reversing it.

❓ Predict Output

intermediate

Output of reversed() with join for string reversal

What is printed by this code that reverses a string using reversed() and join()?

DSA Python

s = "world"
reversed_s = ''.join(reversed(s))
print(reversed_s)

A world

B Error

C ['d', 'l', 'r', 'o', 'w']

D dlrow

💡 Hint

reversed() returns an iterator that goes backwards over the string.

reversed(s) returns characters from the end to the start. Joining them with an empty string creates the reversed string.

❓ Predict Output

advanced

Output of manual string reversal using a loop

What is the output of this code that reverses a string by building a new string in a loop?

DSA Python

s = "abcde"
result = ""
for char in s:
    result = char + result
print(result)

A Error

B abcde

C edcba

D aebcd

💡 Hint

Each new character is added before the existing result string.

By adding each character before the current result, the string is reversed step by step.

❓ Predict Output

advanced

Output of in-place reversal using list conversion

What is printed after reversing a string by converting it to a list and swapping characters in place?

DSA Python

s = "python"
chars = list(s)
left, right = 0, len(chars) - 1
while left < right:
    chars[left], chars[right] = chars[right], chars[left]
    left += 1
    right -= 1
reversed_s = ''.join(chars)
print(reversed_s)

A nohtyp

B python

C Error

D nothyp

💡 Hint

Swapping characters from ends towards the center reverses the list.

Swapping characters at left and right indices moves characters to reverse the list in place.

🧠 Conceptual

expert

Time complexity of different string reversal methods

Which string reversal approach has the best time complexity for reversing a string of length n?

A Converting string to list and swapping characters in place, O(n) time

B Using reversed() and join(), which is O(n) due to join optimization

C Using a loop that concatenates characters to a new string in each iteration, O(n²) time

D Using slicing s[::-1] which creates a new reversed string in O(n) time

💡 Hint

Consider how string concatenation inside loops affects time complexity.

Concatenating strings in a loop is costly because strings are immutable, causing O(n²) time. In-place list swapping and slicing are O(n). reversed() with join() is also O(n) because join is optimized.