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.

Example 1:

    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.

Example 2:

    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.

Example 3:

    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.

Constraints:

- `n == nums.length`
- `1 <= n <= 104`
- `0 <= nums[i] <= n`
- All the numbers of `nums` are unique.

Follow up: Could you implement a solution using only `O(1)` extra space complexity and `O(n)` runtime complexity?