HDU 1255 覆盖的面积 ——（线段树+扫描线）

　　又做了一题扫描线以后对节点的覆盖标记理解的更加深刻了。

　　代码如下：

 #include <stdio.h>
 #include <algorithm>
 #include <string.h>
 #define t_mid (l+r>>1)
 #define ls (o<<1)
 #define rs (o<<1|1)
 #define lson ls,l,t_mid
 #define rson rs,t_mid+1,r
 using namespace std;
 const int N =  + ;
 const int MAX = N * ;
 const double eps = 1e-;

 struct seg
 {
     double x1,x2,y;
     int d;
     bool operator < (const seg & temp) const
     {
         return std::fabs(y-temp.y) <= eps ? d > temp.d : y < temp.y;
     }
 }g[N<<];
 int n,tot,ptot;
 int lazy[MAX<<];
 double pos[MAX],c[MAX<<],cc[MAX<<];
 void add(double x1,double x2,double y,int d)
 {
     tot++;
     g[tot] = {x1,x2,y,d};
 }
 void read()
 {
     tot = ptot = ;
     scanf("%d",&n);
     for(int i=;i<=n;i++)
     {
         double x1,y1,x2,y2;
         scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
         add(x1,x2,y1,);
         add(x1,x2,y2,-);
         pos[++ptot] = x1;
         pos[++ptot] = x2;
     }
     sort(g+,g++tot);
     sort(pos+,pos++ptot);
     int m = ;
     for(int i=;i<=ptot;i++)
     {
         if(std::fabs(pos[i]-pos[i-]) > eps)
         {
             pos[++m] = pos[i];
         }
     }
     ptot = m;
 }

 int Find(double x)
 {
     int L = , R = ptot;
     while(L <= R)
     {
         int mid = L + R >> ;
         if(std::fabs(pos[mid]-x) <= eps) return mid;
         else if(pos[mid] < x) L = mid + ;
         else R = mid - ;
     }
     return -;
 }

 void pushup(int o,int l,int r)
 {
     if(lazy[o] > ) c[o] = pos[r] - pos[l-];
     else if(l == r) c[o] = ;
     else c[o] = c[ls] + c[rs];
     /*************************/
     if(lazy[o] >= ) cc[o] = pos[r] - pos[l-];
     else if(l == r) cc[o] = ;
     else if(lazy[o] == ) cc[o] = c[ls] + c[rs];
     else cc[o] = cc[ls] + cc[rs];
 }

 void update(int o,int l,int r,int ql,int qr,int f)
 {
     if(ql == l && qr == r)
     {
         lazy[o] += f;
         pushup(o,l,r);
         return ;
     }
     if(qr <= t_mid) update(lson,ql,qr,f);
     else if(ql > t_mid) update(rson,ql,qr,f);
     else
     {
         update(lson,ql,t_mid,f);
         update(rson,t_mid+,qr,f);
     }
     pushup(o,l,r);
 }

 void solve()
 {
     memset(lazy,,sizeof(lazy));
     memset(c,,sizeof(c));
     memset(cc,,sizeof(cc));
     double ans = ;
     for(int i=;i<=tot;)
     {
         int j = i;
         while(j <= tot && std::fabs(g[i].y-g[j].y) <= eps)
         {
             int L = Find(g[j].x1);
             int R = Find(g[j].x2);
             /*
             下面的L要加1是因为两个点重合但是其实线段是不重合的，
             所以线段树内不能让他们管辖同一块地方
             而pushup里面再L减1是因为，计算两点之间的距离要用原来的点算
             */
             update(,,MAX,L+, R, g[j].d);
             j++;
         }
         if(j <= tot) ans += 1.0*(g[j].y-g[i].y) * cc[];
         i = j;
     }
     printf("%.2f\n",ans);
 }

 int main()
 {
     int T;
     scanf("%d",&T);
     while(T--)
     {
         read();
         solve();
     }
     return ;
 }