# 99 questions/Solutions/62B

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< 99 questions | Solutions(Difference between revisions)

(rewrite a little shorter and (imho) clearer) |
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Line 5: | Line 5: | ||

<haskell> |
<haskell> |
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atlevel :: Tree a -> Int -> [a] |
atlevel :: Tree a -> Int -> [a] |
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− | atlevel t level = loop t 1 |
+ | atlevel Empty _ = [] |

− | where |
+ | atlevel (Branch v l r) n |

− | loop Empty _ = [] |
+ | | n == 1 = [v] |

− | loop (Branch a l r) n |
+ | | n > 1 = atlevel l (n-1) ++ atlevel r (n-1) |

− | | n == level = [a] |
+ | | otherwise = [] |

− | | otherwise = loop l (n+1) ++ loop r (n+1) |
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</haskell> |
</haskell> |
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## Revision as of 06:35, 22 November 2010

Collect the nodes at a given level in a list

A node of a binary tree is at level N if the path from the root to the node has length N-1. The root node is at level 1. Write a predicate atlevel/3 to collect all nodes at a given level in a list.

atlevel :: Tree a -> Int -> [a] atlevel Empty _ = [] atlevel (Branch v l r) n | n == 1 = [v] | n > 1 = atlevel l (n-1) ++ atlevel r (n-1) | otherwise = []

Another possibility is to decompose the problem:

levels :: Tree a -> [[a]] levels Empty = repeat [] levels (Branch a l r) = [a] : zipWith (++) (levels l) (levels r) atlevel :: Tree a -> Int -> [a] atlevel t n = levels t !! (n-1)