Reverse a String

🏷️ Category: Strings | 🎯 Difficulty: Easy | 🔥 Frequency: ★★★★★

📋 Problem Statement

Write a function that takes a string and returns it reversed.

📥 Input  → "hello"    📤 Output → "olleh"
📥 Input  → "Python"   📤 Output → "nohtyP"
📥 Input  → "abcde"    📤 Output → "edcba"

📥 Input  → "hello"    📤 Output → "olleh"
📥 Input  → "Python"   📤 Output → "nohtyP"
📥 Input  → "abcde"    📤 Output → "edcba"

🧠 What the Interviewer is Testing

Concept	Why It Matters
Python string slicing	Core Python knowledge
String immutability	Shows depth of understanding
Pythonic thinking	Clean code vs verbose loop

⚠️ Key fact: Strings in Python are immutable — you cannot reverse them in place. You must return a new string.

💡 Approach

Use Python’s slice notation s[::-1]

Start from end of string
Step backwards by -1
Returns a brand new reversed string ✅

✅ Optimal Solution

def reverse_string(s: str) -> str:
    # Slice with step -1 reverses the string
    return s[::-1]

# ── Test Cases ─────────────────────────────────
print(reverse_string("hello"))    # ✅ "olleh"
print(reverse_string("Python"))   # ✅ "nohtyP"
print(reverse_string(""))         # ✅ ""
print(reverse_string("a"))        # ✅ "a"

def reverse_string(s: str) -> str:
    # Slice with step -1 reverses the string
    return s[::-1]

# ── Test Cases ─────────────────────────────────
print(reverse_string("hello"))    # ✅ "olleh"
print(reverse_string("Python"))   # ✅ "nohtyP"
print(reverse_string(""))         # ✅ ""
print(reverse_string("a"))        # ✅ "a"

🔄 Alternative Approaches

# Using reversed() + join
def reverse_string_v2(s: str) -> str:
    return ''.join(reversed(s))

# Using a loop (verbose — avoid in interviews)
def reverse_string_v3(s: str) -> str:
    result = ''
    for char in s:
        result = char + result
    return result

# Using reversed() + join
def reverse_string_v2(s: str) -> str:
    return ''.join(reversed(s))

# Using a loop (verbose — avoid in interviews)
def reverse_string_v3(s: str) -> str:
    result = ''
    for char in s:
        result = char + result
    return result

📊 Complexity Analysis

┌─────────────┬────────────────────────────────────┐
│ Time  O(n)  │ Must visit each character once     │
│ Space O(n)  │ New string is created in memory    │
└─────────────┴────────────────────────────────────┘

┌─────────────┬────────────────────────────────────┐
│ Time  O(n)  │ Must visit each character once     │
│ Space O(n)  │ New string is created in memory    │
└─────────────┴────────────────────────────────────┘

🎤 How to Explain in the Interview

“Strings in Python are immutable so I can’t reverse in place. The cleanest approach is slice notation s[::-1] where -1 step means we start from the end and move backwards. This runs in O(n) time and O(n) space.”

🔁 Common Follow-up Questions

Q1 → Can you do it without slicing?

return ''.join(reversed(s))

return ''.join(reversed(s))

Q2 → What if input is None?

if not s:
    return s
return s[::-1]

if not s:
    return s
return s[::-1]

Q3 → Reverse words in a sentence?

def reverse_words(s: str) -> str:
    return ' '.join(s.split()[::-1])
# "Hello World Python" → "Python World Hello"

def reverse_words(s: str) -> str:
    return ' '.join(s.split()[::-1])
# "Hello World Python" → "Python World Hello"

📁 Set 1 of 4 — Python Interview Prep Series — devprepguide.com