freecodecamp--freecodecamp/curriculum/challenges/english/blocks/daily-coding-challenges-python/68af0687ef34c76c28ffa54f.md

id, title, challengeType, dashedName

id	title	challengeType	dashedName
68af0687ef34c76c28ffa54f	Challenge 34: Missing Numbers	29	challenge-34

--description--

Given an array of integers from 1 to n, inclusive, return an array of all the missing integers between 1 and n (where n is the largest number in the given array).

• The given array may be unsorted and may contain duplicates.
• The returned array should be in ascending order.
• If no integers are missing, return an empty array.

--hints--

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

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

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

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

find_missing_numbers([1, 10]) should return [2, 3, 4, 5, 6, 7, 8, 9].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(find_missing_numbers([1, 10]), [2, 3, 4, 5, 6, 7, 8, 9])`)
}})

find_missing_numbers([10, 1, 10, 1, 10, 1]) should return [2, 3, 4, 5, 6, 7, 8, 9].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(find_missing_numbers([10, 1, 10, 1, 10, 1]), [2, 3, 4, 5, 6, 7, 8, 9])`)
}})

find_missing_numbers([3, 1, 4, 1, 5, 9]) should return [2, 6, 7, 8].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(find_missing_numbers([3, 1, 4, 1, 5, 9]), [2, 6, 7, 8])`)
}})

find_missing_numbers([1, 2, 3, 4, 5, 7, 8, 9, 10, 12, 6, 8, 9, 3, 2, 10, 7, 4]) should return [11].

({test: () => { runPython(`
from unittest import TestCase
TestCase().assertEqual(find_missing_numbers([1, 2, 3, 4, 5, 7, 8, 9, 10, 12, 6, 8, 9, 3, 2, 10, 7, 4]), [11])`)
}})

--seed--

--seed-contents--

def find_missing_numbers(arr):

    return arr

--solutions--

def find_missing_numbers(arr):
    if not arr:
        return []
    
    n = max(arr)
    seen = set(arr)
    result = [i for i in range(1, n + 1) if i not in seen]
    return result