hoj3465

和计算几何相关的求逆序数，我是用树状数组做的，也可以用归并排序。 注意，由于是开区间所以读入区间后要将区间向内收缩eps.这道题还比较考察浮点数的相关知识。浮点数比较a<b要写成a+eps < b.

#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
using namespace std;

#define maxn 50005
#define eps 1.0e-8

struct Highway
{
    double a, b;
} way[maxn];

int n, c[maxn], tot, add, totls;
double ls[maxn * 2], l, r;
long long ans;

void input()
{
    scanf("%lf%lf", &l, &r);
    l += eps;
    r -= eps;
    tot = 0;
    add = 0;
    totls = 0;
    for (int i = 0; i < n; i++)
    {
        double x1, x2, y1, y2;
        scanf("%lf%lf%lf%lf", &x1, &y1, &x2, &y2);
        if (abs(x1 - x2) > eps)
        {
            way[tot].a = y2- (y2-y1)/(x2 - x1) * (x2 - l);
            way[tot].b = y2 + (y2- y1)/(x2-x1) * (r - x2);
            ls[totls++] = way[tot].b;
            tot++;
        }
        else if (x1 + eps < r && l + eps < x1)
            add++;
    }
}

int binarysearch(double a)
{
    int l = 0, r = totls - 1;
    int mid;
    while (l < r)
    {
        mid = (l + r) / 2;
        if (ls[mid] + eps < a)
            l = mid + 1;
        else
            r = mid;
    }
    return l;
}

bool cmp(const Highway &a, const Highway &b)
{
    if (abs(a.a - b.a) > eps)
        return a.a + eps < b.a;
    return a.b + eps < b.b;
}

long long cal(int a)
{
    long long sum = 0;
    while (a > 0)
    {
        sum += c[a];
        a -= (a & -a);
    }
    return sum;
}

void modify(int a, int x)
{
    while (a <= tot)
    {
        c[a] += x;
        a += (a & -a);
    }
}

void work()
{
    ans = 0;
    for (int i = 0; i < tot; i++)
    {
        int temp = binarysearch(way[i].b) + 1;
        ans += i - cal(temp);
        modify(temp, 1);
    }
}

int main()
{
    //freopen("D:\\t.txt", "r", stdin);
    while (scanf("%d", &n)!= EOF)
    {
        memset(c, 0, sizeof(c));
        input();
        sort(way, way + tot, cmp);
        sort(ls, ls + totls);
        work();
        printf("%I64d\n", ans + tot * add);
    }
    return 0;
}