freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-python/6821ebee237de8297eaee796.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
6821ebee237de8297eaee796	Challenge 20: Array Duplicates	29	challenge-20

--description--

Given an array of integers, return an array of integers that appear more than once in the initial array, sorted in ascending order. If no values appear more than once, return an empty array.

• Only include one instance of each value in the returned array.

--hints--

find_duplicates([1, 2, 3, 4, 5]) should return [].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(find_duplicates([1, 2, 3, 4, 5]), [])`)
}})

find_duplicates([1, 2, 3, 4, 1, 2]) should return [1, 2].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(find_duplicates([1, 2, 3, 4, 1, 2]), [1, 2])`)
}})

find_duplicates([2, 34, 0, 1, -6, 23, 5, 3, 2, 5, 67, -6, 23, 2, 43, 2, 12, 0, 2, 4, 4]) should return [-6, 0, 2, 4, 5, 23].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(find_duplicates([2, 34, 0, 1, -6, 23, 5, 3, 2, 5, 67, -6, 23, 2, 43, 2, 12, 0, 2, 4, 4]), [-6, 0, 2, 4, 5, 23])`)
}})

--seed--

--seed-contents--

def find_duplicates(arr):

    return arr

--solutions--

def find_duplicates(arr):
    seen = set()
    duplicates = set()

    for num in arr:
        if num in seen:
            duplicates.add(num)
        else:
            seen.add(num)

    return sorted(duplicates)