首先我对着第一篇题解写了发凸包。

#include <bits/stdc++.h>
using namespace std;
#define int long long
inline int read()
{
	int sum = 0, nega = 1;
	char ch = getchar();
	while (ch > '9'||ch < '0')
	{
	    if (ch == '-') nega = -1;
		ch = getchar();
	}
	while (ch <= '9' && ch >= '0') sum = sum * 10 + ch - '0', ch = getchar();
	return sum * nega;
}
const int N = 1e5 + 9;
const double eps = 1e-10;
int n;
struct point
{
	double x, y;
}p[N], sta[N];
int top = 0;
inline double check(point a1, point a2, point b1, point b2) 
{
	return (a2.x - a1.x) * (b2.y - b1.y) - (b2.x - b1.x) * (a2.y - a1.y);
}
inline double dis(point a1, point a2) 
{
	return sqrt((a1.x - a2.x) * (a1.x - a2.x) + (a1.y - a2.y) * (a1.y - a2.y));
}
inline bool cmp(point x, point y) 
{
	double qwq = check(p[1], x, p[1], y);
	if(qwq > eps) return 1;
	if(qwq > -eps && dis(p[1], x) < dis(p[1], y)) return 1;
	return 0; 
}
signed main()
{
	n = read();
	for (int i = 1; i <= n; i++)
		scanf("%lf %lf", &p[i].x, &p[i].y);
	for (int i = 2; i <= n; i++) 
		if(p[1].y - p[i].y > eps) swap(p[1], p[i]);
	sort(p + 2, p + n + 1, cmp);
	sta[++top] = p[1];
	for (int i = 2; i <= n; i++)
	{
		while(top > 1 && check(sta[top - 1], sta[top], sta[top], p[i]) < -eps) top--; 
		sta[++top] = p[i];
	} 
	sta[++top] = p[1];
	double ans = 0;
	for (int i = 1; i < top; i++) ans = ans + dis(sta[i], sta[i + 1]);
	printf("%.2lf\n", ans);
	return 0;
}

结果挂成了 $91$。

然后把

if(qwq > -eps && dis(p[1], x) < dis(p[1], y)) return 1;

改成了

if(qwq > -eps && dis(p[0], x) < dis(p[0], y)) return 1;

就过了。

但是感觉排序的时候应该按照到第一个点的距离来排吧

求大佬解答是为什么