"""
268. Missing Number
src: https://leetcode.com/problems/missing-number/

Given an array nums containing n distinct numbers in the range [0, n], return the only number in the range that is missing from the array.

---

Examples:

Input: nums = [3,0,1]
Output: 2
Explanation: n = 3 since there are 3 numbers, so all numbers are in the range [0,3]. 2 is the missing number in the range since it does not appear in nums.

Input: nums = [0,1]
Output: 2
Explanation: n = 2 since there are 2 numbers, so all numbers are in the range [0,2]. 2 is the missing number in the range since it does not appear in nums.

Input: nums = [9,6,4,2,3,5,7,0,1]
Output: 8
Explanation: n = 9 since there are 9 numbers, so all numbers are in the range [0,9]. 8 is the missing number in the range since it does not appear in nums.

Input: nums = [0]
Output: 1
Explanation: n = 1 since there is 1 number, so all numbers are in the range [0,1]. 1 is the missing number in the range since it does not appear in nums.
"""


def missing_number(nums: list[int]) -> int:
    range_count = len(nums) + 1

    return sum(range(0, range_count)) - sum(nums)


def test_missing_number():
    assert missing_number([3, 0, 1]) == 2
    assert missing_number([0, 1]) == 2
    assert missing_number([9, 6, 4, 2, 3, 5, 7, 0, 1]) == 8
    assert missing_number([0]) == 1
