HDU-1542 Atlantis(线段树扫描线)

There are several ancient Greek texts that contain descriptions of the fabled island Atlantis. Some of these texts even include maps of parts of the island. But unfortunately, these maps describe different regions of Atlantis. Your friend Bill has to know the total area for which maps exist. You (unwisely) volunteered to write a program that calculates this quantity.
Input
The input file consists of several test cases. Each test case starts with a line containing a single integer n (1<=n<=100) of available maps. The n following lines describe one map each. Each of these lines contains four numbers x1;y1;x2;y2 (0<=x1

#include<bits/stdc++.h>
using namespace std;
const int maxn=1e3+10;
struct edge
{
    double x,yu,yd;
    int flag;
    edge() {};
    edge(double a,double b,double c,int d)
    {
        x=a;
        yd=b;
        yu=c;
        flag=d;
    }
    bool operator <(const edge &a)
    {
        return x<a.x;
    }
} a[maxn];
int n;
struct node
{
    int l,r,cnt;
    double len,ren,dis;
} tre[maxn<<2];
double y[maxn];
void build(int l,int r,int rt)
{
    tre[rt].l=l;
    tre[rt].r=r;
    tre[rt].len=y[l];
    tre[rt].ren=y[r];
    tre[rt].cnt=0;
    if(tre[rt].l+1==tre[rt].r) return;
    int mid=(l+r)>>1;
    build(l,mid,rt<<1);
    build(mid,r,rt<<1|1);
}
void cal_dis(int rt)
{
    if(tre[rt].cnt>0) tre[rt].dis=tre[rt].ren-tre[rt].len;///若覆盖计数大于0，该区间的覆盖面积为区间长度
    else if(tre[rt].r-tre[rt].l==1)  tre[rt].dis=0;///叶子节点
    else tre[rt].dis=tre[rt<<1].dis+tre[rt<<1|1].dis;///不是叶子节点，通过向上传递，两个子节点的dis之和即更大区间的dis
    return ;
}
void update(edge line,int rt)
{
    if(tre[rt].len==line.yd&&tre[rt].ren==line.yu)
    {
        tre[rt].cnt+=line.flag;
        cal_dis(rt);
        return ;
    }
    if(line.yu<=tre[rt<<1].ren) update(line,rt<<1);
    else if(line.yd>=tre[rt<<1|1].len) update(line,rt<<1|1);
    else
    {
        edge tmp=line;
        tmp.yu=tre[rt<<1].ren;
        update(tmp,rt<<1);
        tmp=line;
        tmp.yd=tre[rt<<1|1].len;
        update(tmp,rt<<1|1);
    }
    cal_dis(rt);
    return;
}
int main()
{
    double x1,y1,x2,y2;
    int cas=1;
    while(scanf("%d",&n)!=EOF&&n)
    {
        int num=1;
        if(n==0)break;
        for(int i=1; i<=n; i++)
        {
            scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
            y[num]=y1;
            a[num++]=edge(x1,y1,y2,1);
            y[num]=y2;
            a[num++]=edge(x2,y1,y2,-1);
        }
        double ans=0;
        sort(a+1,a+num);
        sort(y+1,y+num);
        build(1,num-1,1);
        update(a[1],1);
        for(int i=2; i<num; i++)
        {
            ans+=tre[1].dis*(a[i].x-a[i-1].x);
            update(a[i],1);
        }
        printf("Test case #%d\n",cas++);
        printf("Total explored area: %.2f\n\n",ans);
    }
}