poj 1228 凸包

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

const double eps = 1e-8;
const double PI = acos(-1.0);
const double INF = 1000000000000000.000;

struct Point{
double x,y;
Point(double x=0, double y=0) : x(x),y(y){ }    //构造函数
};
typedef Point Vector;

Vector operator + (Vector A , Vector B){return Vector(A.x+B.x,A.y+B.y);}
Vector operator - (Vector A , Vector B){return Vector(A.x-B.x,A.y-B.y);}
Vector operator * (double p,Vector A){return Vector(A.x*p,A.y*p);}
Vector operator / (Vector A , double p){return Vector(A.x/p,A.y/p);}

bool operator < (const Point& a,const Point& b){
return a.x < b.x ||( a.x == b.x && a.y < b.y);
}

inline int dcmp(double x){
if(fabs(x) < eps) return 0;
else              return x < 0 ? -1 : 1;
}
bool operator == (const Point& a, const Point& b){
return dcmp(a.x - b.x) == 0 && dcmp(a.y - b.y) == 0;
}

///向量（x,y）的极角用atan2(y,x);
inline double Dot(Vector A, Vector B){ return A.x*B.x + A.y*B.y; }
inline double Length(Vector A)    { return sqrt(Dot(A,A)); }
inline double Angle(Vector A, Vector B)  { return acos(Dot(A,B) / Length(A) / Length(B)); }
inline double Cross(Vector A, Vector B)  { return A.x*B.y - A.y * B.x; }

//凸包：
/**Andrew算法思路：首先按照先x后y从小到大排序（这个地方没有采用极角逆序排序，所以要进行两次扫描），删除重复的点后得到的序列p1,p2.....,然后把p1和p2放到凸包中。从p3开始，当新的
点在凸包“前进”方向的左边时继续，否则依次删除最近加入凸包的点，直到新点在左边；**/

//Goal[]数组模拟栈的使用；
int ConvexHull(Point* P,int n,Point* Goal){
sort(P,P+n);
int m = unique(P,P+n) - P;    //对点进行去重；
int cnt = 0;
for(int i=0;i<m;i++){       //求下凸包；
while(cnt>1 && dcmp(Cross(Goal[cnt-1]-Goal[cnt-2],P[i]-Goal[cnt-2])) <= 0)  cnt--;   //如果希望在凸包的边上有输入点，把<= 换成 <.
Goal[cnt++] = P[i];
}
int temp = cnt;
for(int i=m-2;i>=0;i--){     //逆序求上凸包；
while(cnt>temp && dcmp(Cross(Goal[cnt-1]-Goal[cnt-2],P[i]-Goal[cnt-2])) <= 0) cnt--;
Goal[cnt++] = P[i];
}
if(cnt > 1) cnt--;  //减一为了去掉首尾重复的；
return cnt;
}

bool IsPointOnSegment(Point P,Point A,Point B){
return dcmp(Cross(A-P,B-P)) == 0 && dcmp(Dot(P-A,P-B)) < 0;
}

/*************************************分 割 线*****************************************/

const int maxn = 1005;

Point P[maxn];
Point Goal[maxn];

int main()
{
freopen("E:\\acm\\input.txt","r",stdin);

int T;
cin>>T;

while(T--){
int n;
cin>>n;

for(int i=0;i<n;i++){
scanf("%lf %lf",&P[i].x,&P[i].y);
}

int cnt = ConvexHull(P,n,Goal);

if(cnt <= 2 || cnt*2 > n || n < 6){
printf("NO\n");
continue;
} cout<<cnt<<"  "<<Goal[cnt].x<<"  "<<Goal[cnt].y<<endl;
Goal[cnt] = Goal[0];
int i;
for(i=0;i<cnt;i++){
bool flag = false;
for(int j=0;j<n;j++){
// if(j == i || j == i+1) continue;    加了这句就WA 7 次，痛苦啊
if(IsPointOnSegment(P[j],Goal[i],Goal[i+1])){
flag = true;
}
}
if(!flag) break;
}
if(i<cnt) printf("NO\n");
else      printf("YES\n");
}
}