USACO5.1凸包參考代碼

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cmath>
using namespace std;
const int maxn=10000+5;
struct node{
	double x,y;
}a[maxn];
int stack[maxn],top;
bool cmp(node p,node q){return p.x<q.x;}
double dist(int u,int v){
	return sqrt((a[u].x-a[v].x)*(a[u].x-a[v].x)+(a[u].y-a[v].y)*(a[u].y-a[v].y));
}
bool pd(int u,int v,int w){
	return (a[u].y-a[v].y)*(a[v].x-a[w].x)>(a[v].y-a[w].y)*(a[u].x-a[v].x);
}
int main(){
	int i,j,k,m,n;
	cin>>n;
	for(i=1;i<=n;i++)cin>>a[i].x>>a[i].y;
	sort(a+1,a+n+1,cmp);	
	top=2;
	stack[1]=1;stack[2]=2;
	for(i=3;i<=n;i++){
		while(top>1 && !pd(i,stack[top],stack[top-1]))top--;
		stack[++top]=i;	
	}		
	double ans=0;
	for(i=1;i<top;i++)ans+=dist(stack[i],stack[i+1]);
	top=2;
	stack[1]=1;stack[2]=2;
	for(i=3;i<=n;i++){
		while(top>1 && pd(i,stack[top],stack[top-1]))top--;
		stack[++top]=i;	
	}		
	for(i=1;i<top;i++)ans+=dist(stack[i],stack[i+1]);
	printf("%.2lf\n",ans);
	return 0;
}