freeCodeCamp/curriculum/challenges/english/99-dev-playground/daily-coding-challenges-python/python-challenge-20.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
6821ecb5237de8297eaee7a2	Python Challenge 20: Array Duplicates	29	python-challenge-20

--description--

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