Practice - 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.

For example, if $nums = [3, 0, 1]$, then $n = 3$ and the missing number is $2$.

Hint: use the sum formula $1 + 2 + ... + n = \frac{n(n+1)}{2}$. The missing number equals the expected sum minus the actual sum.