Sum of Squares/Square of Sum Difference
Implementation is good indeed, but I have some reservations calling it fast. Its complexity is O(n). As usual (including, but not limited to, Project Euler), an ultimate optimizer is math. Recall that


  $$ 1 + 2 + \ldots + n = \frac{n(n+1)}{2}$$


and


  $$ 1^2 + 2^2 + \ldots + n^2 = \frac{n(n+1)(2n+1)}{6}$$


so the result


  $$ \frac{n^2(n+1)^2}{4} - \frac{n(n+1)(2n+1)}{6} = \frac{(n-1)n(n+1)(3n+2)}{12}$$


can be achieved in O(1) time.