HDU - 3465 Life is a Line

1.题面

http://acm.hdu.edu.cn/showproblem.php?pid=3465

2.题意

给你一堆直线，求在指定区间内，这里有多少对相交线段

3.思路

求出直线在两个端点的值后，使用归并排序求逆序数

PS:值得注意的坑点，

1.这道题即使取消cin与stdio输入输出相关性，使用cin，cout也会超时

2.排序时，当横坐标相等时，要考虑纵坐标的影响

4.代码

/*****************************************************************
> File Name: Cpp_Acm.cpp
> Author: Uncle_Sugar
> Mail: uncle_sugar@qq.com
> Created Time: 2016年07月30日 星期六 15时58分46秒
*****************************************************************/
# include <cstdio>
# include <cstring>
# include <cctype>
# include <cmath>
# include <cstdlib>
# include <climits>
# include <iostream>
# include <iomanip>
# include <set>
# include <map>
# include <vector>
# include <stack>
# include <queue>
# include <algorithm>
using namespace std;

const int debug = 1;
const int size = 10 + 50000 ;
const int INF = INT_MAX>>1;
const double eps = 1e-6;
typedef long long ll;

int tot = 0;
int num[size];

template<typename T, typename _Compare>
inline ll merge(T *host, T *guest, int len, int wide, _Compare comp){
int l_p,r_p,gst_p;
int start,mid,end;
ll ret = 0;
for (start=0;start<len;start=end){
l_p = gst_p = start;
r_p = mid = start + (wide>>1) < len? start + (wide>>1): len;
end = start + wide < len? start + wide: len;
while (l_p<mid&&r_p<end){
if (comp(host[l_p],host[r_p])){
guest[gst_p++] = host[l_p++];
ret += r_p - mid;
}else {
guest[gst_p++] = host[r_p++];
}
}
while (l_p<mid) {
guest[gst_p++] = host[l_p++];
ret += r_p - mid;
}
while (r_p<end){
guest[gst_p++] = host[r_p++];
}
}
return ret;
}

template<typename T, typename _Compare>
ll mergesort(T *host,int len,_Compare comp){
ll ret = 0;
T *guest = new T[len];
for (int wide=1;wide<len;){
ret += merge(host,guest,len,wide<<=1,comp);
ret += merge(guest,host,len,wide<<=1,comp);
}
delete guest;
return ret;
}

struct val{
double l,r;
}v[size];

bool cmp(const val& c1,const val& c2){
if (c1.l != c2.l)
return c1.l < c2.l;
return c1.r < c2.r;
}

bool cmp2(const val& c1,const val& c2){
if (c1.r != c2.r)
return c1.r < c2.r;
return c1.l < c2.l;
}

int main()
{
std::ios::sync_with_stdio(false);
int i,j;
int n;
double lb,rb;
while (~scanf("%d",&n)){
scanf("%lf%lf",&lb,&rb);
double x1,x2,y1,y2;
int t = 0;
for (i=0;i<n;i++){
scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);
if (fabs(x1-x2)<eps){
x1 -= eps;
x2 += eps;
//# i--;n--;
//# if (lb<x1&&x1<rb){
//# t++;
//# }
//# continue;
}
double k = (y1 - y2)/(x1 - x2);
double b = y1 - k*x1;
v[i].l = k*lb + b;
v[i].r = k*rb + b;
}
sort(v,v+n,cmp);
printf("%lld\n",mergesort(v,n,cmp2)+t*n);
}
return 0;
}