凸包模板（以洛谷P2742为例）

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
#define maxn 100005
using namespace std;
double X,Y;
struct node
{
    double x,y;
}e[maxn],stk[maxn];
bool cmp(node a,node b)
{
    if (a.y==b.y) return a.x<b.x;
    return a.y<b.y;
}
bool cmp2(node a,node b)//极角排序 
{
    if (atan2(a.y-Y,a.x-X)==atan2(b.y-Y,b.x-X)) return a.x<b.x;
    return (atan2(a.y-Y,a.x-X))<(atan2(b.y-Y,b.x-X));
}
double cross(node a,node b,node c)
{
    node ab,ac;
    ab.x=b.x-a.x;
    ab.y=b.y-a.y;
    ac.x=c.x-a.x;
    ac.y=c.y-a.y;
    return ab.x*ac.y-ab.y*ac.x;
}
double getdis(node a,node b)
{
    node ab;
    ab.x=b.x-a.x;
    ab.y=b.y-a.y;
    return sqrt(ab.x*ab.x+ab.y*ab.y);
}
int main()
{
    int i,n,top;
    double ans;
    scanf("%d",&n);
    for (i=0;i<n;i++) scanf("%lf%lf",&e[i].x,&e[i].y);
    if (n<3) 
    {
        if (n==1) printf("0\n");
        else if (n==2) printf("%.2lf\n",getdis(e[0],e[1]));
        return 0;
    }
    sort(e,e+n,cmp);
    top=-1;
    stk[++top]=e[0];
    X=e[0].x;Y=e[0].y;
    sort(e+1,e+n,cmp2);
    stk[++top]=e[1];
    for (i=2;i<n;i++)
    {
        while (cross(stk[top-1],stk[top],e[i])<0) top--;
        stk[++top]=e[i];
    }
    ans=0;
    for (i=1;i<=top;i++)
    {
        ans+=getdis(stk[i],stk[i-1]);
    } 
    ans+=getdis(stk[0],stk[top]);
    printf("%.2lf\n",ans); 
    return 0;
} 

凸包模板