O( 2n + 2(n-1) + 2(n-2) + ... + 2(2) + 2(1) )

          2n + 2(n-1) + 2(n-2) + ... + 2(2) + 2(1)
        = 2(n + n-1 + n-2 + ... + 2 + 1)
        = 2(n*(n+1)/2)
        = n^2 + n

= O( n^2 + n )
= O( n^2 )

##</code><code>##

  O( n * 2n )
= O( 2n^2 )
= O( n^2 )