[USACO07OPEN]城市的地平线City Horizon

题目描述

Farmer John has taken his cows on a trip to the city! As the sun sets, the cows gaze at the city horizon and observe the beautiful silhouettes formed by the rectangular buildings.

The entire horizon is represented by a number line with $N$ ($1 ≤ N ≤ 40,000$) buildings. Building i’s silhouette has a base that spans locations $A_i$ through $B_i$ along the horizon (1≤