https://ac.nowcoder.com/acm/contest/328/F

C++版本一

題解：計算幾何裸題

參考文章：https://blog.csdn.net/weixin_43272781/article/details/85800560

/* *@Author: STZG *@Language: C++ */ #include <bits/stdc++.h> #include<iostream> #include<algorithm> #include<cstdlib> #include<cstring> #include<cstdio> #include<string> #include<vector> #include<bitset> #include<queue> #include<deque> #include<stack> #include<cmath> #include<list> #include<map> #include<set> //#define DEBUG #define RI register int using namespace std; typedef long long ll; //typedef __int128 lll; const int N=100000+10; const int MOD=1e9+7; const double PI = acos(-1.0); const double EXP = 1E-8; const int INF = 0x3f3f3f3f; int t,n,m,k,q; double ans; struct node{double x,y;}e[N]; double a[N]; char str; int main() { #ifdef DEBUGfreopen("input.in", "r", stdin);//freopen("output.out", "w", stdout); #endifscanf("%d%d",&n,&q);if(n==3){printf("%.6f",ans);return 0;}for(int i=1;i<=n;i++){scanf("%lf%lf",&e[i].x,&e[i].y);}for(int i=1;i<=n;i++){if(n==1)a[i]=a[i-1]+e[i].y*e[n].x-e[i].x*e[n].y;elsea[i]=a[i-1]+e[i].y*e[i-1].x-e[i].x*e[i-1].y;}double sum=fabs(a[n]/2);//cout<<sum<<endl;int s,t;while(q--){scanf("%d%d",&s,&t);if(s>t)swap(s,t);if(abs(s-t)==1||s==1&&t==n)continue;double sumtmp=fabs((a[t]-a[s]+e[s].y*e[t].x-e[s].x*e[t].y)/2);ans=max(ans,min(sumtmp,sum-sumtmp));}printf("%.6f",ans);//cout << "Hello world!" << endl;return 0; }

C++版本二

題解：

計算幾何
只要叉積維護一下前綴和就好了。

#include <cstdio> #include <bits/stdc++.h> #include <map> #include <cstring> #include <algorithm> using namespace std; #define mst(a,b) memset((a),(b),sizeof(a)) #define rush() int T;scanf("%d",&T);while(T--)typedef long long ll; const int maxn = 200005; const ll INF = 1e18; const ll mod=1e9+7; const double eps = 1e-9;int n,m;struct node {double x,y; }a[maxn];double sum[maxn];double cross(node a,node b,node c) {return (b.x-a.x)*(c.y-a.y)-(c.x-a.x)*(b.y-a.y); }int main() {scanf("%d%d",&n,&m);for(int i=1;i<=n;i++){scanf("%lf%lf",&a[i].x,&a[i].y);}node zero;zero.x=0;zero.y=0;for(int i=1;i<=n;i++){double ans=cross(zero,a[i],a[(i==n?1:i+1)]);sum[i]=sum[i-1]+ans;}for(int i=n+1;i<=2*n;i++){sum[i]=sum[i-1]+sum[i-n];}double Sum=fabs(sum[n])/2.0;double cnt=0;for(int i=0;i<m;i++){int ss,tt;scanf("%d%d",&ss,&tt);if(ss>tt) swap(ss,tt);double ans=sum[tt-1]-sum[ss-1];ans+=cross(zero,a[tt],a[ss]);ans=fabs(ans)/2.0;cnt=max(cnt,min(ans,Sum-ans));}printf("%.15f\n",cnt); }

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