The challenge

Write a function that takes a shuffled list of unique numbers from 1 to n with one element missing (which can be any number including n). Return this missing number.

Note: huge lists will be tested.

Examples:

[1, 3, 4]  =>  2
[1, 2, 3]  =>  4
[4, 2, 3]  =>  1

The solution in Python code

Option 1:

def find_missing_number(a):
    n = len(a) + 1
    return n * (n + 1) // 2 - sum(a)

Option 2:

def find_missing_number(nums):
    return sum(range(1,len(nums)+2))-sum(nums)

Option 3:

def find_missing_number(numbers):
    if numbers == []:return 1
    diff = list(set(range(1, len(numbers)+1))- set(numbers))
    if diff == []:return max(numbers)+1
    return diff[0]

Test cases to validate our solution

test.assert_equals(find_missing_number([2, 3, 4]), 1)
test.assert_equals(find_missing_number([1, 3, 4]), 2)
test.assert_equals(find_missing_number([1, 2, 4]), 3)
test.assert_equals(find_missing_number([1, 2, 3]), 4)