说明：  贪吃蛇游戏是一个深受人们喜爱的游戏，一条蛇在密闭的围墙内，在围墙内随机出现一个食物，通过按键盘上的四个光标键控制蛇向上下左右四个方向移动。
(Snake game is a very popular game, a snake in a closed wall, the walls appear randomly a food move to the four directions, four cursor keys on your keyboard to control the snake )