Create README.md

Author: lukhol

binaryTree/README.md

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+### Binary Tree / Binary Search Tree
+
+** Count number of nodes **
+```kotlin
+class Solution {
+    // I need to traverse the tree in in-order manner
+    fun numberOfNodes(root: TreeNode?): Int {
+        fun dfs(node: TreeNode?, counter: Int): Int {
+            if (node == null) return counter
+            val left = dfs(node.left, counter)   
+            return left + dfs(node.right, left + 1)
+        }
+
+        return dfs(root, 0)
+    }
+}
+```
+
+** Find kth element in the tree **
+```kotlin
+class Solution {
+    fun kthSmallest(root: TreeNode?, k: Int): Int {
+        if (root == null) return -1
+        var kthSmallestElement = 0
+
+        fun dfs(node: TreeNode?, counter: Int): Int {
+            if (node == null) return counter
+            val left = dfs(node.left, counter)
+            val mid = left + 1
+            if (mid == k) {
+                kthSmallestElement = node.`val`
+            }
+            val right = dfs(node.right, mid)
+            return right
+        }
+
+        dfs(root, 0)
+
+        return kthSmallestElement
+    }
+}
+```