# 99 questions/61 to 69

### From HaskellWiki

These are Haskell translations of Ninety Nine Lisp Problems.

If you want to work on one of these, put your name in the block so we know someone's working on it. Then, change n in your block to the appropriate problem number, and fill in the <Problem description>,<example in lisp>,<example in Haskell>,<solution in haskell> and <description of implementation> fields.

## 1 Problem 61

Count the leaves of a binary tree

A leaf is a node with no successors. Write a predicate count_leaves/2 to count them.

Example: % count_leaves(T,N) :- the binary tree T has N leaves Example in Haskell: > count_leaves (Branch 1 (Branch 2 Nil (Branch 4 Nil Nil)) (Branch 2 Nil Nil)) 2

Solution:

data Tree a = Nil | Branch a (Tree a) (Tree a) deriving (Show, Eq) count_leaves Nil = 0 count_leaves (Branch a Nil Nil) = 1 count_leaves (Branch a left right) = count_leaves left + count_leaves right

## 2 Problem 61A

Collect the leaves of a binary tree in a list

A leaf is a node with no successors. Write a predicate leaves/2 to collect them in a list.

Example: % leaves(T,S) :- S is the list of all leaves of the binary tree T Example in Haskell: > leaves (Branch 1 (Branch 2 Nil (Branch 4 Nil Nil)) (Branch 2 Nil Nil)) [Branch 4 Nil Nil, Branch 2 Nil Nil]

Solution:

leaves Nil = [] leaves b@(Branch a Nil Nil) = [b] leaves (Branch a left right) = leaves left ++ leaves right

## 3 Problem 62

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## 4 Problem 62B

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## 5 Problem 63

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## 6 Problem 64

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## 7 Problem 65

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## 8 Problem 66

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## 9 Problem 67

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## 10 Problem 68

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## 11 Problem 69

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