Difference between revisions of "99 questions/54A to 60"
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== Binary trees == 
== Binary trees == 

−  A binary tree is either empty or it is composed of a root element and two successors, which are binary trees themselves. 
+  A binary tree is either empty or it is composed of a root element and two successors, which are binary trees themselves. In Haskell, we can characterize binary trees with a datatype definition: 
−  
−  https://prof.ti.bfh.ch/hew1/informatik3/prolog/p99/p67.gif 

−  
−  In Haskell, we can characterize binary trees with a datatype definition: 

<haskell> 
<haskell> 

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</haskell> 
</haskell> 

−  This says that a < 
+  This says that a <code>Tree</code> of type <code>a</code> consists of either an <code>Empty</code> node, or a <code>Branch</code> containing one value of type <code>a</code> with exactly two subtrees of type <code>a</code>. 
Given this definition, the tree in the diagram above would be represented as: 
Given this definition, the tree in the diagram above would be represented as: 

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== Problem 54A == 
== Problem 54A == 

−  +  * ''Check whether a given term represents a binary tree'' 

In Prolog or Lisp, one writes a predicate to do this. 
In Prolog or Lisp, one writes a predicate to do this. 

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Nonsolution: 
Nonsolution: 

−  Haskell's type system ensures that all terms of type < 
+  Haskell's type system ensures that all terms of type <code>Tree a</code> are binary trees: it is just not possible to construct an invalid tree with this type. Hence, it is redundant to introduce a predicate to check this property: it would always return <code>True</code>. 
== Problem 55 == 
== Problem 55 == 

−  +  * ''Construct completely balanced binary trees'' 

In a completely balanced binary tree, the following property holds for every node: The number of nodes in its left subtree and the number of nodes in its right subtree are almost equal, which means their difference is not greater than one. 
In a completely balanced binary tree, the following property holds for every node: The number of nodes in its left subtree and the number of nodes in its right subtree are almost equal, which means their difference is not greater than one. 

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== Problem 56 == 
== Problem 56 == 

−  +  * ''Symmetric binary trees'' 

Let us call a binary tree symmetric if you can draw a vertical line through the root node and then the right subtree is the mirror image of the left subtree. Write a predicate symmetric/1 to check whether a given binary tree is symmetric. Hint: Write a predicate mirror/2 first to check whether one tree is the mirror image of another. We are only interested in the structure, not in the contents of the nodes. 
Let us call a binary tree symmetric if you can draw a vertical line through the root node and then the right subtree is the mirror image of the left subtree. Write a predicate symmetric/1 to check whether a given binary tree is symmetric. Hint: Write a predicate mirror/2 first to check whether one tree is the mirror image of another. We are only interested in the structure, not in the contents of the nodes. 

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== Problem 57 == 
== Problem 57 == 

−  +  * ''Binary search trees (dictionaries)'' 

Use the predicate add/3, developed in chapter 4 of the course, to write a predicate to construct a binary search tree from a list of integer numbers. 
Use the predicate add/3, developed in chapter 4 of the course, to write a predicate to construct a binary search tree from a list of integer numbers. 

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== Problem 58 == 
== Problem 58 == 

−  +  * ''Generateandtest paradigm'' 

Apply the generateandtest paradigm to construct all symmetric, completely balanced binary trees with a given number of nodes. 
Apply the generateandtest paradigm to construct all symmetric, completely balanced binary trees with a given number of nodes. 

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== Problem 59 == 
== Problem 59 == 

−  +  * ''Construct heightbalanced binary trees'' 

In a heightbalanced binary tree, the following property holds for every node: The height of its left subtree and the height of its right subtree are almost equal, which means their difference is not greater than one. 
In a heightbalanced binary tree, the following property holds for every node: The height of its left subtree and the height of its right subtree are almost equal, which means their difference is not greater than one. 

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== Problem 60 == 
== Problem 60 == 

−  +  * ''Construct heightbalanced binary trees with a given number of nodes'' 

Consider a heightbalanced binary tree of height H. What is the maximum number of nodes it can contain? 
Consider a heightbalanced binary tree of height H. What is the maximum number of nodes it can contain? 

−  Clearly, MaxN = 2 
+  Clearly, MaxN = 2<sup>H</sup>  1. However, what is the minimum number MinN? This question is more difficult. Try to find a recursive statement and turn it into a function <code>minNodes</code> that returns the minimum number of nodes in a heightbalanced binary tree of height H. 
−  On the other hand, we might ask: what is the maximum height H a heightbalanced binary tree with N nodes can have? Write a function < 
+  On the other hand, we might ask: what is the maximum height H a heightbalanced binary tree with N nodes can have? Write a function <code>maxHeight</code> that computes this. 
Now, we can attack the main problem: construct all the heightbalanced binary trees with a given number of nodes. Find out how many heightbalanced trees exist for N = 15. 
Now, we can attack the main problem: construct all the heightbalanced binary trees with a given number of nodes. Find out how many heightbalanced trees exist for N = 15. 
Revision as of 21:19, 5 April 2021
This is part of NinetyNine Haskell Problems, based on NinetyNine Prolog Problems.
Binary trees
A binary tree is either empty or it is composed of a root element and two successors, which are binary trees themselves. In Haskell, we can characterize binary trees with a datatype definition:
data Tree a = Empty  Branch a (Tree a) (Tree a)
deriving (Show, Eq)
This says that a Tree
of type a
consists of either an Empty
node, or a Branch
containing one value of type a
with exactly two subtrees of type a
.
Given this definition, the tree in the diagram above would be represented as:
tree1 = Branch 'a' (Branch 'b' (Branch 'd' Empty Empty)
(Branch 'e' Empty Empty))
(Branch 'c' Empty
(Branch 'f' (Branch 'g' Empty Empty)
Empty))
Since a "leaf" node is a branch with two empty subtrees, it can be useful to define a shorthand function:
leaf x = Branch x Empty Empty
Then the tree diagram above could be expressed more simply as:
tree1' = Branch 'a' (Branch 'b' (leaf 'd')
(leaf 'e'))
(Branch 'c' Empty
(Branch 'f' (leaf 'g')
Empty)))
Other examples of binary trees:
 A binary tree consisting of a root node only
tree2 = Branch 'a' Empty Empty
 An empty binary tree
tree3 = Empty
 A tree of integers
tree4 = Branch 1 (Branch 2 Empty (Branch 4 Empty Empty))
(Branch 2 Empty Empty)
Problem 54A
 Check whether a given term represents a binary tree
In Prolog or Lisp, one writes a predicate to do this.
Example in Lisp:
* (istree (a (b nil nil) nil)) T * (istree (a (b nil nil))) NIL
Nonsolution:
Haskell's type system ensures that all terms of type Tree a
are binary trees: it is just not possible to construct an invalid tree with this type. Hence, it is redundant to introduce a predicate to check this property: it would always return True
.
Problem 55
 Construct completely balanced binary trees
In a completely balanced binary tree, the following property holds for every node: The number of nodes in its left subtree and the number of nodes in its right subtree are almost equal, which means their difference is not greater than one.
Write a function cbaltree to construct completely balanced binary trees for a given number of nodes. The predicate should generate all solutions via backtracking. Put the letter 'x' as information into all nodes of the tree.
Example:
* cbaltree(4,T). T = t(x, t(x, nil, nil), t(x, nil, t(x, nil, nil))) ; T = t(x, t(x, nil, nil), t(x, t(x, nil, nil), nil)) ; etc......No
Example in Haskell, whitespace and "comment diagrams" added for clarity and exposition:
λ> cbalTree 4
[
 permutation 1
 x
 / \
 x x
 \
 x
Branch 'x' (Branch 'x' Empty Empty)
(Branch 'x' Empty
(Branch 'x' Empty Empty)),
 permutation 2
 x
 / \
 x x
 /
 x
Branch 'x' (Branch 'x' Empty Empty)
(Branch 'x' (Branch 'x' Empty Empty)
Empty),
 permutation 3
 x
 / \
 x x
 \
 x
Branch 'x' (Branch 'x' Empty
(Branch 'x' Empty Empty))
(Branch 'x' Empty Empty),
 permutation 4
 x
 / \
 x x
 /
 x
Branch 'x' (Branch 'x' (Branch 'x' Empty Empty)
Empty)
(Branch 'x' Empty Empty)
]
Problem 56
 Symmetric binary trees
Let us call a binary tree symmetric if you can draw a vertical line through the root node and then the right subtree is the mirror image of the left subtree. Write a predicate symmetric/1 to check whether a given binary tree is symmetric. Hint: Write a predicate mirror/2 first to check whether one tree is the mirror image of another. We are only interested in the structure, not in the contents of the nodes.
Example in Haskell:
λ> symmetric (Branch 'x' (Branch 'x' Empty Empty) Empty)
False
λ> symmetric (Branch 'x' (Branch 'x' Empty Empty) (Branch 'x' Empty Empty))
True
Problem 57
 Binary search trees (dictionaries)
Use the predicate add/3, developed in chapter 4 of the course, to write a predicate to construct a binary search tree from a list of integer numbers.
Example:
* construct([3,2,5,7,1],T). T = t(3, t(2, t(1, nil, nil), nil), t(5, nil, t(7, nil, nil)))
Then use this predicate to test the solution of the problem P56.
Example:
* testsymmetric([5,3,18,1,4,12,21]). Yes * testsymmetric([3,2,5,7,4]). No
Example in Haskell:
λ> construct [3, 2, 5, 7, 1]
Branch 3 (Branch 2 (Branch 1 Empty Empty) Empty) (Branch 5 Empty (Branch 7 Empty Empty))
λ> symmetric . construct $ [5, 3, 18, 1, 4, 12, 21]
True
λ> symmetric . construct $ [3, 2, 5, 7, 1]
True
Problem 58
 Generateandtest paradigm
Apply the generateandtest paradigm to construct all symmetric, completely balanced binary trees with a given number of nodes.
Example:
* symcbaltrees(5,Ts). Ts = [t(x, t(x, nil, t(x, nil, nil)), t(x, t(x, nil, nil), nil)), t(x, t(x, t(x, nil, nil), nil), t(x, nil, t(x, nil, nil)))]
Example in Haskell:
λ> symCbalTrees 5
[Branch 'x' (Branch 'x' Empty (Branch 'x' Empty Empty)) (Branch 'x' (Branch 'x' Empty Empty) Empty),Branch 'x' (Branch 'x' (Branch 'x' Empty Empty) Empty) (Branch 'x' Empty (Branch 'x' Empty Empty))]
Problem 59
 Construct heightbalanced binary trees
In a heightbalanced binary tree, the following property holds for every node: The height of its left subtree and the height of its right subtree are almost equal, which means their difference is not greater than one.
Construct a list of all heightbalanced binary trees with the given element and the given maximum height.
Example:
? hbal_tree(3,T). T = t(x, t(x, t(x, nil, nil), t(x, nil, nil)), t(x, t(x, nil, nil), t(x, nil, nil))) ; T = t(x, t(x, t(x, nil, nil), t(x, nil, nil)), t(x, t(x, nil, nil), nil)) ; etc......No
Example in Haskell:
λ> take 4 $ hbalTree 'x' 3
[Branch 'x' (Branch 'x' Empty Empty) (Branch 'x' Empty (Branch 'x' Empty Empty)),
Branch 'x' (Branch 'x' Empty Empty) (Branch 'x' (Branch 'x' Empty Empty) Empty),
Branch 'x' (Branch 'x' Empty Empty) (Branch 'x' (Branch 'x' Empty Empty) (Branch 'x' Empty Empty)),
Branch 'x' (Branch 'x' Empty (Branch 'x' Empty Empty)) (Branch 'x' Empty Empty)]
Problem 60
 Construct heightbalanced binary trees with a given number of nodes
Consider a heightbalanced binary tree of height H. What is the maximum number of nodes it can contain?
Clearly, MaxN = 2^{H}  1. However, what is the minimum number MinN? This question is more difficult. Try to find a recursive statement and turn it into a function minNodes
that returns the minimum number of nodes in a heightbalanced binary tree of height H.
On the other hand, we might ask: what is the maximum height H a heightbalanced binary tree with N nodes can have? Write a function maxHeight
that computes this.
Now, we can attack the main problem: construct all the heightbalanced binary trees with a given number of nodes. Find out how many heightbalanced trees exist for N = 15.
Example in Prolog:
? count_hbal_trees(15,C). C = 1553
Example in Haskell:
λ> length $ hbalTreeNodes 'x' 15
1553
λ> map (hbalTreeNodes 'x') [0..3]
[[Empty],
[Branch 'x' Empty Empty],
[Branch 'x' Empty (Branch 'x' Empty Empty),Branch 'x' (Branch 'x' Empty Empty) Empty],
[Branch 'x' (Branch 'x' Empty Empty) (Branch 'x' Empty Empty)]]