How to remove duplicates?

intermediate

Remove Duplicates

Remove duplicate values from a list.

Learn to use sets for efficient duplicate detection.

📚 Concepts & Theory

Using Sets

Sets have O(1) lookup:

seen = set()
if x not in seen:
    seen.add(x)

🎯 Your Challenge

Write remove_duplicates that preserves order.

📝 Starter Code

Python

def remove_duplicates(items):
    pass

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

Solution

Python

def remove_duplicates(items):
    seen = set()
    result = []
    for item in items:
        if item not in seen:
            seen.add(item)
            result.append(item)
    return result

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

Explanation

Track seen items with a set.

❓ Frequently Asked Questions

Use set() or dict.fromkeys()