solution.py

# You're working in a number zoo, and it seems that one of the numbers has gone missing!

# Zoo workers have no idea what number is missing, and are too incompetent to figure it out,
# so they're hiring you to do it for them.

# In case the zoo loses another number, they want your program to work regardless of how many numbers there are in total.

# Task:
# 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


def find_missing_number(numbers):
    # return list(set(range(1, len(numbers) + 2)) - set(numbers))[0] - about 10 seconds of all tests
    # return list(set(range(1, len(numbers) + 2)) ^ set(numbers))[0] - about 10 seconds of all tests
    n = len(numbers) + 1
    return n * (n + 1) // 2 - sum(numbers)  # about 6 seconds all tests