$ g++ tictactoe.cc -O3 -o game

$ ./game 
start a new game
choose player O or X
X

AI stage

- - - 
- - - 
- - O 

please enter position 1~9
1

Human stage

X - - 
- - - 
- - O 

AI stage

X - - 
- - - 
O - O 

please enter position 1~9
8

Human stage

X - - 
- - - 
O X O 

AI stage

X - O 
- - - 
O X O 

please enter position 1~9
6

Human stage

X - O 
- - X 
O X O 

AI stage

X - O 
- O X 
O X O 
AI win!
1. press c to change  Difficulty 
2. press x to exit game!
3. anything else to continue