解决方法
这是基本的数学求解线 – 线交叉,检查
tutorial的topcoder：
Line-Line Intersection One of the most common tasks you will find in geometry problems is line intersection. Despite the fact that it is so common,a lot of coders still have trouble with it. The first question is,what form are we given our lines in,and what form would we like them in? Ideally,each of our lines will be in the form Ax+By=C,where A,B and C are the numbers which define the line. However,we are rarely given lines in this format,but we can easily generate such an equation from two points. Say we are given two different points,(x1,y1) and (x2,y2),and want to find A,B and C for the equation above. We can do so by setting A = y2-y1 B = x1-x2 C = A*x1+B*y1
