beginner

Reverse a List

Reverse a list in Python without built-in methods.

List reversal teaches you about mutable vs immutable objects, in-place modifications, and space-time trade-offs. Understanding different reversal methods helps you choose the right approach for your specific use case.

📚 Concepts & Theory

Reversing a list is a fundamental operation in Python that can be accomplished through multiple approaches. Unlike strings, lists are mutable, meaning they can be modified in-place.

Key Distinction: In-Place vs Creating New List

Method 1: Slicing (Creates New List)

original = [1, 2, 3, 4, 5]

reversed_list = original[::-1]  # [5, 4, 3, 2, 1]

# original is unchanged

Method 2: reverse() Method (In-Place)

numbers = [1, 2, 3, 4, 5]

numbers.reverse()  # Modifies the list directly

# numbers is now [5, 4, 3, 2, 1]

Method 3: reversed() Function (Returns Iterator)

original = [1, 2, 3, 4, 5]

reversed_list = list(reversed(original))

Method 4: Manual Loop (Educational)

numbers = [1, 2, 3, 4, 5]

reversed_list = []

for num in numbers:

    reversed_list.insert(0, num)  # Insert at beginning

# Or: reversed_list = [num] + reversed_list

Method 5: Two-Pointer Swap (In-Place)

def reverse_in_place(lst):

    left, right = 0, len(lst) - 1

    while left < right:

        lst[left], lst[right] = lst[right], lst[left]

        left += 1

        right -= 1

Understanding the Trade-offs:

Slicing [::-1]: Most Pythonic, creates new list O(n) space

reverse(): Fastest in-place, modifies original O(1) space

reversed(): Memory efficient iterator, good for large lists

Manual loop: Educational, shows the logic clearly

🎯 Your Challenge

Write a function reverse_list without using reverse() or [::-1].

📝 Starter Code

Python

def reverse_list(items):
    pass

print(reverse_list([1, 2, 3]))

Python has a slicing syntax that can reverse sequences
The syntax is [start:stop:step]
A step of -1 means go backwards
This creates a new list, doesn't modify the original
The simplest solution is just one line: [::-1]

Solution

Python

def reverse_list(items):
    result = []
    for i in range(len(items)-1, -1, -1):
        result.append(items[i])
    return result

print(reverse_list([1, 2, 3]))

Explanation

This solution uses Python's slicing syntax [::-1] to create a new list with elements in reverse order. The step value of -1 tells Python to traverse the list backwards. This creates a shallow copy with reversed order, leaving the original list unchanged. While not the most space-efficient method, it's the most concise and Pythonic. Time complexity: O(n). Space complexity: O(n) for the new list. For in-place reversal, use the reverse() method instead.

⚠️ Common Mistakes to Avoid

Confusing reverse() method (in-place) with reversed() function
Expecting [::-1] to modify the original list
Not assigning the result of [::-1] to a variable
Using reversed() without converting to list
Trying to use string methods on lists

❓ Frequently Asked Questions

reverse() modifies the list in-place and returns None. reversed() returns an iterator without modifying the original.

No, it creates a new list. Use reverse() to modify in-place.

The reverse() method is fastest for in-place reversal. [::-1] is fastest for creating a new reversed list.

Use slicing with indices: lst[2:5][::-1] reverses elements from index 2 to 4.

📚 Concepts & Theory

🎯 Your Challenge

📝 Starter Code

Solution

Explanation

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

🔗 Related Exercises

Python Tuples

Remove Duplicates

Merge Two Sorted Lists

Reverse a String