diff --git a/arduino/bluetooth_arduino.cpp b/arduino/bluetooth_arduino.cpp
new file mode 100644
index 0000000..cc7ded6
--- /dev/null
+++ b/arduino/bluetooth_arduino.cpp
@@ -0,0 +1,47 @@
+// Basic Bluetooth sketch HC-06_01
+// // Connect the Hc-06 module and communicate using the serial monitor
+// //
+// // The HC-06 defaults to AT mode when first powered on.
+// // The default baud rate is 9600
+// // The Hc-06 requires all AT commands to be in uppercase. NL+CR should not be
+// added to the command string
+// //
+//  
+//   
+//   #include <SoftwareSerial.h>
+//   SoftwareSerial BTserial(2, 3); // RX | TX
+//   // Connect the HC-06 TX to the Arduino RX on pin 2. 
+//   // Connect the HC-06 RX to the Arduino TX on pin 3 through a voltage
+//   divider.
+//   // 
+//    
+//     
+//     void setup() 
+//     {
+//         Serial.begin(9600);
+//             Serial.println("Enter AT commands:");
+//              
+//                  // HC-06 default serial speed is 9600
+//                      BTserial.begin(9600);  
+//                      }
+//                       
+//                       void loop()
+//                       {
+//                        
+//                            // Keep reading from HC-06 and send to Arduino
+//                            Serial Monitor
+//                                if (BTserial.available())
+//                                    {  
+//                                            Serial.write(BTserial.read());
+//                                                }
+//                                                 
+//                                                     // Keep reading from
+//                                                     Arduino Serial Monitor
+//                                                     and send to HC-06
+//                                                         if
+//                                                         (Serial.available())
+//                                                             {
+//                                                                     BTserial.write(Serial.read());
+//                                                                         }
+//                                                                          
+//                                                                          }
diff --git a/arduino/bluetooth_serial_test.py b/arduino/bluetooth_serial_test.py
new file mode 100755
index 0000000..918bb4b
--- /dev/null
+++ b/arduino/bluetooth_serial_test.py
@@ -0,0 +1,21 @@
+#!/usr/bin/python
+
+#
+#   - Pair bluetooth useing bluetoothctl.
+#       agent on
+#       pair 20:16:02:14:58:51
+#   - Setup serial connection:
+#       sudo rfcomm bind hci0 20:16:02:14:58:51 1
+#
+#   - For module
+#       sudo apt-get install python-serial
+#
+
+import serial
+from time import sleep
+
+bluetoothSerial = serial.Serial( "/dev/rfcomm0", baudrate=9600 )
+
+bluetoothSerial.write( "Test from python." )
+print bluetoothSerial.readline()
+
diff --git a/puzzles/interviews/training/preorder_tree_construction.cpp.orig b/puzzles/interviews/training/preorder_tree_construction.cpp.orig
new file mode 100644
index 0000000..595e16a
--- /dev/null
+++ b/puzzles/interviews/training/preorder_tree_construction.cpp.orig
@@ -0,0 +1,102 @@
+/*
+VIM: let b:lcppflags="-std=c++11 -O2 -pthread"
+VIM: let b:wcppflags="/O2 /EHsc /DWIN32"
+VIM: let b:argv=""
+*/
+#include <vector>
+<<<<<<< HEAD
+#include <string>
+||||||| merged common ancestors
+=======
+#include <stack>
+>>>>>>> preorder tree construction: much better algorithm. No need to keep root.
+#include <iostream>
+
+struct node {
+    node * left = nullptr;
+    node * right = nullptr;
+    double data = 0;
+
+    node( double d )
+    : data(d)
+    {}
+};
+
+node * prefix_to_tree( const std::vector<double>& v ){
+
+    if ( !v.size() ){
+        return nullptr;
+    }
+
+    node * root = new node(v[0]);
+
+    std::stack<node*> stack;
+    stack.push(root);
+
+    for ( int i = 1; i < v.size(); ) {
+        if ( v[i] < stack.top()->data ){
+            auto n = new node(v[i++]);
+            stack.top()->left = n;
+            stack.push(n);
+        } else {
+            auto c = stack.top();
+            stack.pop();
+            while( !stack.empty() && stack.top()->data < v[i] ){
+                c = stack.top();
+                stack.pop();
+            }
+
+            auto n = new node(v[i++]);
+            c->right = n;
+            stack.push(n);
+        }
+    }
+
+    return root;
+}
+
+void print( node * root, const std::string& level = "" ){
+    if ( root ) {
+        print( root->left, level + "   ");
+        std::cout << level << "- " << root->data << std::endl;
+        print( root->right, level + "   ");
+    }
+}
+
+void traverse( node * root, std::vector<double>& v ){
+    if ( root ) {
+        v.push_back(root->data);
+        traverse(root->left, v);
+        traverse(root->right, v);
+    }
+}
+
+void test( const std::vector<double>& v ){
+    std::cout << "-----------------------------------------------------" << std::endl;
+    auto r = prefix_to_tree( v );
+    print( r );
+    std::vector<double> w;
+    traverse( r, w );
+    if ( v.size() != w.size() ) {
+        std::cout << "v.size() != w.size() => " << v.size() << " != " << w.size() << std::endl;
+    }
+    for (int i = 0; i < v.size(); ++i ){
+        if (v[i]!=w[i]){
+            std::cout << "error at index " << i << " -> "<< v[i] << " != " << w[i] << std::endl;
+        }
+    }
+}
+
+int main( void ) {
+
+    test( {1} );
+    test( {2, 1, 3} );
+    test( {4, 2, 1, 3, 6, 5, 7} );
+    test( {8, 4, 2, 1, 3, 6, 5, 7, 12, 10, 9, 11, 14, 13, 15} );
+    test( {8, 4, 2,    3, 6, 5, 7, 12, 10, 9, 11, 14, 13, 15} );
+    test( {8, 4,          6, 5, 7, 12, 10, 9,     14, 13, 15} );
+    test( {8,                      12, 10, 9, 11, 14, 13,   } );
+    test( {8, 4, 2, 1,    6,       12, 10,    11, 14,     15} );
+
+    return 0;
+}