few changes in docs

Author: eulerkochy

docs/src/avl_tree.md

@@ -4,7 +4,7 @@ DocTestSetup = :(using DataStructures)
 
 # AVL Tree
 
-The `AVLTree` type is an implementation of AVL Tree in Julia. It is a self-balancing binary search tree where balancing occurs based on the difference of height of the left subtree and the right subtree. Operations such as search, insert and delete can be done in `O(log n)` complexity, where `n` is the number of nodes in the `AVLTree`.
+The `AVLTree` type is an implementation of AVL Tree in Julia. It is a self-balancing binary search tree where balancing occurs based on the difference of height of the left subtree and the right subtree. Operations such as search, insert and delete can be done in `O(log n)` complexity, where `n` is the number of nodes in the `AVLTree`. Order-statistics on the keys can also be done in `O(log n)`.
 
 Examples:
 
@@ -18,7 +18,7 @@ julia> for k in 1:2:20
 julia> haskey(tree, 3)
 true
 
-julia> tree[4]
+julia> tree[4] # this operation is O(log n)
 7
 
 julia> for k in 1:2:10

src/avl_tree.jl

@@ -176,9 +176,7 @@ end
 function Base.delete!(tree::AVLTree{K}, d::K) where K
 
     function delete_node!(node::Union{AVLTreeNode, Nothing}, key)
-        if node == nothing || node.data == nothing
-            return nothing
-        elseif key < node.data
+        if key < node.data
             node.leftChild = delete_node!(node.leftChild, key)
         elseif key > node.data
             node.rightChild = delete_node!(node.rightChild, key)

test/runtests.jl

@@ -33,7 +33,7 @@ tests = ["deprecations",
          "ordered_robin_dict",
          "dibit_vector",
          "red_black_tree",
-         "avl_tree",
+         "avl_tree"
         ]
 
 if length(ARGS) > 0