Add support for trees without values

README.md

@@ -43,4 +43,10 @@ BinaryTrees.search(tree, 3) # right node
 # delete nodes from the tree
 BinaryTrees.delete!(tree, 1)
 BinaryTrees.delete!(tree, 3)
+
+# tree that only has keys
+tree = AVLTree{Int}()
+BinaryTrees.insert!(tree, 2) # root node
+BinaryTrees.insert!(tree, 1) # left node
+BinaryTrees.insert!(tree, 3) # right node
 ```

src/avl.jl

@@ -25,10 +25,9 @@ Base.convert(::Type{AVLNode{K,V}}, node::AVLNode) where {K,V} =
 Construct an empty AVL Tree with keys of type `K`
 and values of type `V`.
 
-    AVLTree()
+    AVLTree{K}()
 
-Construct an empty AVL Tree that stores keys and values
-of any type, alias for `AVLTree{Any,Any}()`.
+Construct an empty AVL tree with no values and keys of type `K`.
 
 The keys of AVL Tree  must implement sorting operators (`>`, `<`)
 and comparison operators (`=`, `≠`)
@@ -54,14 +53,20 @@ BinaryTrees.search(tree, 3) # right node
 # delete nodes from the tree
 BinaryTrees.delete!(tree, 1)
 BinaryTrees.delete!(tree, 3)
+
+# tree that only has keys
+tree = AVLTree{Int}()
+BinaryTrees.insert!(tree, 2) # root node
+BinaryTrees.insert!(tree, 1) # left node
+BinaryTrees.insert!(tree, 3) # right node
 ```
 """
 mutable struct AVLTree{K,V} <: BinaryTree
   root::Union{AVLNode{K,V},Nothing}
 end
 
 AVLTree{K,V}() where {K,V} = AVLTree{K,V}(nothing)
-AVLTree() = AVLTree{Any,Any}()
+AVLTree{K}() where {K} = AVLTree{K,Nothing}()
 
 search(tree::AVLTree{K}, key::K) where {K} = _search(tree, key)
 
@@ -70,6 +75,8 @@ function insert!(tree::AVLTree{K}, key::K, value) where {K}
   tree
 end
 
+insert!(tree::AVLTree{K,Nothing}, key::K) where {K} = insert!(tree, key, nothing)
+
 function delete!(tree::AVLTree{K}, key::K) where {K}
   tree.root = _delete!(tree.root, key)
   tree

src/binarytree.jl

@@ -5,7 +5,7 @@
 """
     BinaryNode
 
-Binary tree node with key, value and optional left and right children.
+Binary tree node with key and optional value, left and right children.
 """
 abstract type BinaryNode end
 
@@ -19,7 +19,7 @@ key(node::BinaryNode) = node.key
 """
     BinaryTrees.value(node)
 
-Value of the `node`.
+Value of the `node`, if it does not exist, `nothing` is returned.
 """
 value(node::BinaryNode) = node.value
 
@@ -69,6 +69,10 @@ function search end
 Insert a node into the `tree` with `key` and `value`.
 If a node with `key` already exists, the value
 of the node will be updated.
+
+    BinaryTrees.insert!(tree, key)
+
+Insert a node into the `tree` with `key` and no value.
 """
 function insert! end
 
@@ -117,7 +121,12 @@ AbstractTrees.nodetype(T::Type{<:BinaryNode}) = T
 
 function AbstractTrees.printnode(io::IO, node::BinaryNode)
   ioctx = IOContext(io, :compact => true, :limit => true)
-  show(ioctx, key(node))
-  print(ioctx, " => ")
-  show(ioctx, value(node))
+  val = value(node)
+  if isnothing(val)
+    show(ioctx, key(node))
+  else
+    show(ioctx, key(node))
+    print(ioctx, " => ")
+    show(ioctx, val)
+  end
 end

test/runtests.jl

@@ -156,7 +156,7 @@ const BT = BinaryTrees
     @test tree |> BT.root |> BT.right |> BT.left |> BT.key == 3
 
     # tree that accepts any types
-    tree = AVLTree()
+    tree = AVLTree{Int,Any}()
     BT.insert!(tree, 2, 'A')
     BT.insert!(tree, 1, 1.1)
     BT.insert!(tree, 3, "test")
@@ -174,6 +174,25 @@ const BT = BinaryTrees
     BT.delete!(tree, 2)
     @test isnothing(BT.root(tree))
 
+    # tree without values
+    tree = AVLTree{Int}()
+    BT.insert!(tree, 2)
+    BT.insert!(tree, 1)
+    BT.insert!(tree, 3)
+    @test isnothing(BT.value(BT.search(tree, 2)))
+    @test isnothing(BT.value(BT.search(tree, 1)))
+    @test isnothing(BT.value(BT.search(tree, 3)))
+    BT.delete!(tree, 3)
+    @test !isnothing(BT.root(tree))
+    @test !isnothing(BT.left(BT.root(tree)))
+    @test isnothing(BT.right(BT.root(tree)))
+    BT.delete!(tree, 1)
+    @test !isnothing(BT.root(tree))
+    @test isnothing(BT.left(BT.root(tree)))
+    @test isnothing(BT.right(BT.root(tree)))
+    BT.delete!(tree, 2)
+    @test isnothing(BT.root(tree))
+
     # AbstractTrees interface
     tree = AVLTree{Int,Int}()
     BT.insert!(tree, 3, 30)
@@ -210,5 +229,15 @@ const BT = BinaryTrees
     │  └─ 1 => 10
     └─ 4 => 40
        └─ 5 => 50"""
+
+    tree = AVLTree{Int}()
+    BT.insert!(tree, 2)
+    BT.insert!(tree, 1)
+    BT.insert!(tree, 3)
+    @test sprint(show, MIME("text/plain"), tree) == """
+    AVLTree
+    2
+    ├─ 1
+    └─ 3"""
   end
 end