线段树的节点对应的是一个区间 而不是区间的端点。。

#include <bits/stdc++.h>
#define lson (rt << 1)
#define rson (rt << 1 | 1)
#define mid (L + R >> 1)
using namespace std;
const int N = 1e5+10;
vector<double> pos;
struct Seg
{
    double l,r,h;
    int f;
    Seg(double a,double b,double c,int d)
    {
        l = a, r = b;
        h = c, f = d;
    }
    bool operator < (const Seg &b)const
    {
        return h < b.h;
    }
};
struct node
{
    int cnt;
    double len;
}t[N<<4];
vector<Seg> segs;
int n;
int getid(double x)
{
    return lower_bound(pos.begin(),pos.end(),x) - pos.begin() + 1;
}
void pushdown(int L,int R,int rt)
{
    if(t[rt].cnt) t[rt].len = pos[R]-pos[L-1];
    else if(L == R) t[rt].len = 0;
    else t[rt].len = t[lson].len + t[rson].len;
}
void update(int l,int r,int v,int L = 1,int R = pos.size(),int rt = 1)
{
    if(l > r) return;
    if(l <= L && R <= r)
    {
        t[rt].cnt += v;
        pushdown(L,R,rt);
        return;
    }
    if(l <= mid) update(l,r,v,L,mid,lson);
    if(r >  mid) update(l,r,v,mid+1,R,rson);
    pushdown(L,R,rt);
}
int main()
{
    int id = 1;
    double x1,y1,x2,y2;
    while(cin >> n && n)
    {
        segs.clear(); pos.clear();
        for(int i = 0; i < n; ++i)
        {
            cin >> x1 >> y1 >> x2 >> y2;
            segs.push_back(Seg(x1,x2,y1,1));
            segs.push_back(Seg(x1,x2,y2,-1));
            pos.push_back(x1); pos.push_back(x2);
        }
        sort(pos.begin(),pos.end());
        sort(segs.begin(),segs.end());
        pos.erase(unique(pos.begin(),pos.end()),pos.end());
        double ans = 0;
        for(int i = 0; i < segs.size(); ++i)
        {
            int l = getid(segs[i].l), r = getid(segs[i].r)-1;
            update(l,r,segs[i].f);
            ans += t[1].len * (segs[i+1].h - segs[i].h);
        }
        printf("Test case #%d\nTotal explored area: %.2lf\n\n",id++,ans);
    }
}