Want to save binary tree to disk for “20 questions” game

void encodeState(OutputStream out,Node n) {
        if(n==null) {
            out.write("[null]");
        } else {
           out.write("{");
           out.write(n.nodeDetails());
           encodeState(out, n.yesNode());
           encodeState(out, n.noNode());
           out.write("}");
        }
  }

F

F
B

F
 B  G

F
 B  G
A

1,2,3,"Does it have wings?"
2,0,0,"a bird"
3,4,0,"Does it purr?"
4,0,0,"a cat"

<node identifier>
node data
[<yes child identfier>
  yes child]
[<no child identifier>
  no child]
<end of node identifier>

<parent>,<relation>,<child>

"Is it Red", "yes", "does it have wings"
"Is it Red", "no" , "does it swim"

[0x1111111 "Is It Red"           => [ 'yes' => 0xF752347 , 'no' => 0xFF6F664 ],
 0xF752347 "does it have wings"  => [ 'yes' => 0xFFFFFFF , 'no' => 0x2222222 ],
 0xFF6F664 "does it swim"        => [ 'yes' => "I Dont KNOW :( " , ... etc etc ]