本人的代码过了样例，提交的时候输出全是负数，代码如下：

#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<cstring>
#include<cmath>
#include<iomanip>
#include<stack>
#include<queue>
#include<vector>
using namespace std;
struct node{
	int x,y,z;
};
node zb[50010];
int cmp(node x,node y)
{
	return x.z<y.z;
}
int main()
{
	int n;
	double ans=0;
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
	{
		scanf("%d%d%d",&zb[i].x,&zb[i].y,&zb[i].z);
	}
	sort(zb+1,zb+n+1,cmp);
	for(int i=1;i<n;i++)
	{
		ans+=sqrt((zb[i+1].x-zb[i].x)*(zb[i+1].y-zb[i].y)+(zb[i+1].y-zb[i].y)*(zb[i+1].z-zb[i].z)+(zb[i+1].z-zb[i].z)*(zb[i+1].z-zb[i].z));
	}
	printf("%.3lf",ans);
	return 0;
}