不多说...凸包网上解法很多,这个是用graham的极角排序,也就是算导上的那个解法

其实其他方法随便乱搞都行...我只是测一下模板...

struct POINT{    double x,y;    POINT(double _x = 0, double _y = 0):x(_x),y(_y){};};POINT p[MAXN],s[MAXN];double dist(POINT p1,POINT p2){    return(sqrt((p1.x-p2.x) * (p1.x-p2.x) + (p1.y-p2.y) * (p1.y-p2.y)));}double multiply(POINT sp,POINT ep,POINT op){    return (sp.x-op.x) * (ep.y-op.y) - (ep.x-op.x) * (sp.y-op.y);}bool ptcmp(POINT a,POINT b){    // 极角排序cmp p[]为全局变量    if(multiply(a,b,p[0]) == 0) return dist(p[0],a) < dist(p[0],b);    return (multiply(a,b,p[0]) > 0);}int Graham_scan(POINT p[],POINT s[],int n){ //  返回凸包点的个数    int i,k = 0,top = 2;    for(i = 1; i < n ; i++)     //  取y最小且x最小的点为凸包起点        if((p[i].y < p[k].y) || ((p[i].y == p[k].y) && (p[i].x < p[k].x)))            k = i;    swap(p[0],p[k]);            //  起点设置为p[0]    sort(p+1,p+n,ptcmp);        //  极角排序    for(i = 0 ; i < 3 ; i++)        s[i] = p[i];            //  前三个点入栈    for(i = 3; i < n ; i++){        while(multiply(p[i],s[top],s[top-1]) >= 0)            top--;        s[++top] = p[i];    }    return top + 1;}int main(){    int a,b,n;    double r;    while(~scanf("%d%lf",&n,&r)){        for(int i = 0 ; i < n ; i++)            scanf("%lf%lf",&p[i].x,&p[i].y);        int len = Graham_scan(p,s,n);        double sum = dist(s[len-1],s[0]);        for(int i = 0 ; i < len-1 ; i++){            sum += dist(s[i],s[i+1]);        }        sum += (PI*r*2);        cout<
     
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