Given any string of N (>=5) characters, you are asked to form the
characters into the shape of U. For example, "helloworld" can
be printed as:
h d
e l
l r
lowo
That is, the characters must be printed in the original order,
starting top-down from the left vertical line with n1 characters, then
left to right along the bottom line with n2 characters, and finally
bottom-up along the vertical line with n3 characters. And more, we
would like U to be as squared as possible -- that is, it must be
satisfied that n1 = n3 = max { k| k <= n2 for all 3 <= n2 <= N
} with n1 + n2 + n3 - 2 = N.