二维凸包板子。。有时间会补总结的。

#include 
    
     #include 
     
      #include 
      
       using namespace std;const int MAXN = 10010;struct point{    double x, y;}p[MAXN];int cmp1(const point a, const point b){    return a.x == b.x ? a.y < b.y : a.x < b.x;}double k(point a, point b){    return a.x == b.x ? 1e18 : (b.y - a.y) / (b.x - a.x);}double dis(point a, point b){    double px = b.x - a.x, py = b.y - a.y;    return sqrt(px * px + py * py);}int n, top, st[MAXN];double ans;int main(){    scanf("%d", &n);    for(int i = 1; i <= n; ++i)       scanf("%lf%lf", &p[i].x, &p[i].y);    sort(p + 1, p + n + 1, cmp1);    for(int i = 1; i <= n; ++i){       while((top > 1 && k(p[st[top - 1]], p[i]) < k(p[st[top - 1]], p[st[top]]))) --top;       st[++top] = i;    }    for(int i = 2; i <= top; ++i)       ans += dis(p[st[i]], p[st[i - 1]]);    top = 0;    for(int i = 1; i <= n; ++i){       while((top > 1 && k(p[st[top - 1]], p[i]) > k(p[st[top - 1]], p[st[top]]))) --top;       st[++top] = i;    }    for(int i = 2; i <= top; ++i)       ans += dis(p[st[i]], p[st[i - 1]]);    printf("%.2lf\n", ans);    return 0;}