poj 2074 Line of Sight（视线问题，求直线与线段的交点及判断相交）

Line of Sight

Time Limit: 1000MS		Memory Limit: 30000K
Total Submissions: 2634		Accepted: 813

Description

An architect is very proud of his new home and wants to be sure it can be seen by people passing by his property line along the street. The property contains various trees, shrubs, hedges, and other obstructions that may block the view. For the purpose of this problem, model the house, property line, and obstructions as straight lines parallel to the x axis: 

To satisfy the architect's need to know how visible the house is, you must write a program that accepts as input the locations of the house, property line, and surrounding obstructions and calculates the longest continuous portion of the property line from which the entire house can be seen, with no part blocked by any obstruction.