说明：  2) 设计编写一捉迷藏游戏：当用户鼠标靠近目标圆时，圆圈跑出鼠标捕捉点。 提示：先画出一个圆，监听鼠标移动事件，鼠标进入圈内，则删除该圆，在原有范围外，重新画一个同样大小的圆。
(2) prepare a hide and seek game design: When a user mouse close to the target circle, the circle ran the mouse capture point. Hint: Draw a circle, listen mouse events, mouse enters the circle, then remove the circle, outside the scope of the original and re-draw a circle the same size.)