题意
给定一个手掌的边界点(20个),你需要判断这是左手还是右手；给定的手掌可能是斜的而且大小也不一定
思路
找到手掌中最长的那一条线段，然后判断题目给出的点事顺时针还是逆时针，然后用最长的线段和右侧一
条线段的比例来判断究竟是左手还是右手,（记得控制精度，因为给定的x，y精确到1e-6,所以eps直接取1e-6）
代码
#pragma GCC optimize(2)
#include<unordered_map>
#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<string>
#include<vector>
#include<queue>
#include<stack>
#include<cmath>
#include<map>
#include<set>
#define Buff ios::sync_with_stdio(false)
#define rush() int Case = 0; int T; cin >> T; while(T--)
#define rep(i, a, b) for(int i = a; i <= b; i ++)
#define per(i, a, b) for(int i = a; i >= b; i --)
#define reps(i, a, b) for(int i = a; b; i ++)
#define clc(a, b) memset(a, b, sizeof(a))
#define Buff ios::sync_with_stdio(false)
#define readl(a) scanf("%lld", &a)
#define readd(a) scanf("%lf", &a)
#define readc(a) scanf("%c", &a)
#define reads(a) scanf("%s", a)
#define read(a) scanf("%d", &a)
#define lowbit(n) (n&(-n))
#define pb push_back
#define lson rt<<1
#define rson rt<<1|1
#define ls lson, l, mid
#define rs rson, mid+1, r
#define y second
#define x first
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int>PII;
const int mod = 1e9+7;
const double eps = 1e-6;
const int N = 1e6+7;
const double PI = acos(-1);
int dcmp(double x)
{
if(fabs(x) < eps) return 0;
else return x < 0 ? -1 : 1;
}
struct Point
{
double x;
double y;
Point(double x=0, double y=0):x(x), y(y) {}
};
typedef Point Vector;
double operator * (const Vector & v, const Vector & w) {return v.x * w.x + v.y * w.y;}
double operator ^ (const Vector & v, const Vector & w) {return v.x * w.y - v.y * w.x;}
double length(Vector v) {return sqrt(v * v);}
Vector operator - (const Vector & v, const Vector & w) {return Vector(v.x - w.x, v.y - w.y);}
void print(string s, Point t)
{
cout << s <<": ("<< t.x <<" "<< t.y <<")" <<endl;
}
Point p[30];
int main()
{
rush()
{
for(int i = 0; i < 20; i ++)
{
double x, y;
cin >> x >> y;
p[i] = {x, y};
}
double len = 0.0;
int pos = 0;
for(int i = 0; i < 20; i ++)
{
Vector v = p[i]-p[(i+1)%20];
if(length(v) > len)
{
len = length(p[i]-p[(i+1)%20]);
pos = i;
}
}
Point a = p[pos], b = p[(pos+1)%20], c = p[(pos+2)%20];
Vector v = b-a, w = c-a, u = c-b;
double temp1 = 9.0/8.0, temp2 = 3.0/2.0;
// cout <<"pos: "<< pos <<endl;
// cout << length(v)/length(u) <<endl;
if(dcmp(v^w) > 0)
{
// cout << "colock!" <<endl;
double k = length(v)/length(u);
if(!dcmp(k-temp1))
{
puts("right");
}
else if(!dcmp(k-temp2))
{
puts("left");
}
}
else
{
// cout << "uncolock!" <<endl;
double k = length(v)/length(u);
if(!dcmp(k-temp2))
{
puts("right");
}
else if(!dcmp(k-temp1))
{
puts("left");
}
}
}
return 0;
}