# Algorithm exercise

making exercises to prepare a test I'm having problems to understand 2 questions, the questions are:

 how many are the leafs of a decisional tree associated to any algorithm for
the search problem  in a ordered set?


for this question I have 2 set of answer where, for every set, 1 is right, looking at the solution I found this, but I'm not able to understand why the right answers are all C.

    1.                 a) Θ(n log n)    b) Θ(log n) *c) Ω(n!)   d) O(n!)

2.                 a) Θ(n log n)    b) Θ(log n) *c) Ω(n)    d) Θ(n!)


The other question, referred to the first one is:

  and in a non-ordered one?


And here I can't see what does it change with the number of leafs in the ordered case.

I'm sorry if this question violates the rules, in the faqs, I read here http://meta.stackexchange.com/questions/10811/how-to-ask-and-answer-homework-questions and it seems to be possible to make these kind of question.

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## migrated from cstheory.stackexchange.comNov 27 '12 at 19:06

This question came from our site for theoretical computer scientists and researchers in related fields.

Is that the whole question? What kind of search and decision tree is it talking about? It would help if you posted something to show you've tried to solve the problem yourself. Do you have any ideas and why it might be the right answer, or did you think another answer is correct? – fgb Nov 27 '12 at 16:49
the quesion is all there, there's not written about any specific tree, the problem is to understand the question and what does it mean, for example if I have N elements, are not the leafs N+1 for not found? – newbie Nov 27 '12 at 17:16
Which answer is associated with which question? – Joe Nov 27 '12 at 20:39
both answers 1. and 2. are associated to the first question – newbie Nov 27 '12 at 20:56

$\Omega(n!)$ is not a reasonable answer. The decision tree for the search problem on the ordered set is just a binary search tree. It has $\Theta(n)$ leaves. Unless they mean something non-standard by "search problem" and "decision tree". These arguments in terms of number of leaves in the decision tree are typically used in lower bound arguments. Wikipedia has an (incomplete) discussion of the decision tree model.
In the unordered case, the decision tree still has $\Theta(n)$ leaves, using linear search.