hdu 1392 Surround the Trees(凸包模版)

写得有点圡

水平排序版本：

#include <cmath>
#include <cstdio>
#include <algorithm>
#include <iostream>
using namespace std;
struct point{
    double x,y;
    bool operator < (point &a){
        return y<a.y || (y == a.y && x<a.x);
    }
};
bool mult(point &sp,point &ep,point &op){
    return (sp.x-op.x)*(ep.y-op.y)
        >=(ep.x-op.x)*(sp.y-op.y);
}
double dist(point &a,point &b){
    return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
double graham(point *pnt,int n,point res[]){
    int i,len,k=0,top=1;
    sort(pnt,pnt+n);
    if(n==0) return 0; res[0]=pnt[0];
    if(n==1) return 0; res[1]=pnt[1];
    if(n==2) return dist(pnt[0],pnt[1]); res[2]=pnt[2];
    for(i=2;i<n;i++){
        while(top && mult(pnt[i],res[top],res[top-1])) //左转退栈，上凸包
            top--;
        res[++top]=pnt[i];
    }
    len=top;
    res[++top]=pnt[n-2];
    for(i=n-3;i>=0;i--){
        while(top!=len && mult(pnt[i],res[top],res[top-1]))//下凸包。
            top--;
        res[++top]=pnt[i];
    }
    double area=0;
    for(i=1;i<=top;i++){
        area+=dist(res[i],res[(i+1)>top?1:(i+1)]);
    }
    
    return area;
}
int main(){
    int n;
    point pnt[105],res[105];
    while(scanf("%d",&n) && n){

        for(int i=0;i<n;i++)
            scanf("%lf %lf",&pnt[i].x,&pnt[i].y);
        printf("%.2lf\n",graham(pnt,n,res));
    }
    return 0;
}

极角排序版本：

#include <cstdio>
#include <cstring>
#include <iostream>
#include <cmath>
#include <algorithm>
using namespace std;
typedef struct point{
	double x,y;
	bool operator < (point &a){
		return y<a.y || (y==a.y && x<a.y); //重载小于号
	}
}p;
p temp;
double dist(p &a,p &b){
	return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
}
bool cmp(p &a,p &b){
	double xmt=(a.x-temp.x)*(b.y-temp.y)-(b.x-temp.x)*(a.y-temp.y);
	if(xmt)	//向量不共线就按逆时针旋转
	return xmt>0;

	return dist(a,temp)>dist(b,temp);//向量共线取最长的。
}
bool xmult(p& a,p& b,p& t){ //叉乘大于0 往左转，叉乘小于0往右转，等于0向量共线
	return (a.x-t.x)*(b.y-t.y)>=(b.x-t.x)*(a.y-t.y);
}
void swap(p& a,p &b){
	temp=b;
	b=a;
	a=temp;
}
double graham(p *pnt,int n,p *res){ //Graham扫描法
	if(n==1) return 0; 
	if(n==2) return dist(pnt[0],pnt[1]); 
	int top=-1;
	for(int i=0;i<n;i++){
		while(top>1 && xmult(pnt[i],res[top],res[top-1]))
			top--;
		res[++top] = pnt[i];
	}
	double dis=0;
	for(int j=0;j<=top;j++){
		dis+=dist(res[j],res[(j+1)%(top+1)]);
	}
	return dis;
}
int main(){
	int n;
	p pnt[105],res[105];
	while(scanf("%d",&n) && n){
		int t;
		for(int i=0;i<n;i++){
			scanf("%lf %lf",&pnt[i].x,&pnt[i].y);
			if(!i || pnt[i]<pnt[t]) //找出最小y坐标值并且最靠左的点
				t=i;
		}
		swap(pnt[0],pnt[t]);
		sort(pnt+1,pnt+n,cmp);
		printf("%.2lf\n",graham(pnt,n,res));
	}
	return 0;
}