problems: add inorder bst traversal

Signed-off-by: Matej Focko <mfocko@redhat.com>

problems/binary-tree-inorder-traversal.kt

@@ -0,0 +1,26 @@
+/**
+ * Example:
+ * var ti = TreeNode(5)
+ * var v = ti.`val`
+ * Definition for a binary tree node.
+ * class TreeNode(var `val`: Int) {
+ *     var left: TreeNode? = null
+ *     var right: TreeNode? = null
+ * }
+ */
+class Solution {
+    fun inorderTraversal(root: TreeNode?, values: MutableList<Int>): List<Int> {
+        if (root == null) {
+            return values
+        }
+
+        inorderTraversal(root.left, values)
+        values.add(root.`val`!!)
+        inorderTraversal(root.right, values)
+
+        return values
+    }
+
+    fun inorderTraversal(root: TreeNode?): List<Int>
+        = inorderTraversal(root, mutableListOf<Int>())
+}