Add tests

test/runtests.jl

@@ -2,5 +2,46 @@ using TreeDataStructures
 using Test
 
 @testset "TreeDataStructures.jl" begin
-    # Write your tests here.
+  @testset "AVLTree" begin
+    tree = AVLTree{Int,Int}()
+    tree[1] = 10
+    tree[2] = 20
+    tree[3] = 30
+    @test tree[1] == 10
+    @test tree[2] == 20
+    @test tree[3] == 30
+
+    # value conversion
+    tree = AVLTree{Int,Float64}()
+    tree[1] = 10
+    tree[2] = 20
+    tree[3] = 30
+    @test tree[1] isa Float64
+    @test tree[1] == 10.0
+    @test tree[2] isa Float64
+    @test tree[2] == 20.0
+    @test tree[3] isa Float64
+    @test tree[3] == 30.0
+
+    # tree that accept any types
+    tree = AVLTree()
+    tree[1] = 1.1
+    tree[2] = 'A'
+    tree[3] = "test"
+    @test tree[1] == 1.1
+    @test tree[2] == 'A'
+    @test tree[3] == "test"
+
+    # show
+    tree = AVLTree{Int,Int}()
+    @test sprint(show, MIME("text/plain"), tree) == "AVLTree()"
+    tree[1] = 10
+    tree[2] = 20
+    tree[3] = 30
+    @test sprint(show, MIME("text/plain"), tree) == """
+    AVLTree
+    2 => 20
+    ├─ 1 => 10
+    └─ 3 => 30"""
+  end
 end