程序运行Bresenham的直线算法，所用坐标系为笛卡尔直角坐标系，

起始状态：用户输入所画线段的起点和终点坐标（要求起点横坐标小于终点横坐标）

结束状态：程序打印出所画线段-Bresenham run a straight line algorithm, used for the Cartesian coordinates Cartesian coordinate system, the initial state: the user to enter the painting of the start and finish line coordinates (request less than the end of the starting point of abscissa abscissa) end status: print out the painting process segment