Arduino Button interference
In Arduino, just setting the pin as input already makes it high impedance.  
Writing HIGH on an input pin actually enables its internal pull-up resistor.  
This way, the voltage at the pin is always defined (instead of floating when the switch is open).  
Just search for "pull-up resistor" if you need more information.  
Note that this code:  

pinMode(n, INPUT); // set as input, high-impedance
digitalWrite(n, HIGH); // activate the internal pull-up resistor


is equivalent to this (in recent Arduino IDEs):  

pinMode(n, INPUT_PULLUP)