[Haskell-cafe] Re: Help with Bird problem 4.5.6: sequence of
successive maxima
Benedikt Huber
benjovi at gmx.net
Sun Mar 15 20:17:17 EDT 2009
R J schrieb:
> This Bird problem vexes me, in the first instance because it doesn't
> seem to specify a unique solution:
>
> Given a list xs = [x_1, x_2, . . . , x_n], the sequence of successive
> maxima "ssm xs" is the
> longest subsequence [x_j1, x_j2, x_j3..x_jk] such that j_1 = 1 and j_m <
> j_n => x_jm < x_jn.
> For example, xs = [3, 1, 3, 4, 9, 2, 10, 7] => ssm xs = [3, 4, 9, 10].
> Define "ssm" in terms of "foldl".
Hi,
this problem is a variant of
http://en.wikipedia.org/wiki/Longest_increasing_subsequence
discussed about a year ago:
http://www.mail-archive.com/haskell-cafe@haskell.org/msg39784.html
http://www.mail-archive.com/haskell-cafe@haskell.org/msg39844.html
In the latter mail, Chris Kuklewicz presents an (efficient)
implementation. To solve the problem you stated, simply reuse `lnds`:
> -- ssm [3, 1, 3, 4, 9, 2, 9, 10, 7] = [3,4,9,10]
> ssm (x:xs) = x : lnds (filter (> x) xs)
benedikt
>
> From this specification, I infer:
>
> ssm [] = []
> ssm [1] = [1]
> ssm [1, 2, 3] = [1, 2, 3]
> ssm [1, 0, 3, 2] = [1, 3]
>
> However, what is ssm [1,0,100,2,3,4,5]? Is it [1, 100] or [1, 2, 3, 4,
> 5]? I think the latter, but am not certain. Whichever it is, what's
> the solution?
>
> Thanks.
>
>
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