poj 2349 最小生成树 careful everything！

#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <math.h>

using namespace std;

#define NUM 505

typedef struct{
	int x,y;
}Node;

Node nodes[NUM];

typedef struct{
	int from;
	int to;
	double cost;
}Edge;
int cmp(const void* a,const void * b){
	Edge aa=*((Edge *)a);
	Edge bb=*((Edge *)b);
	if(aa.cost>bb.cost)
		return 1;
	if(aa.cost==bb.cost)
		return 0;
	else 
		return -1;
}
Edge edges[NUM*NUM/2];

double GetLength(double x1,double y1,double x2,double y2){
	double r=sqrt(double(abs(x1-x2)*abs(x1-x2)+abs(y1-y2)*abs(y1-y2)));
	return r;
}
int root[NUM];
void init(){
	for(int k=0;k<NUM;k++){
		root[k]=k;
	}
}
//不要失去警惕
//这里第三行代码错误的写成了
/*
	root[a]=findroot(a);
	所以在poj提交之后，得到了runtime error的错误
*/
int findroot(int a){
	if(a!=root[a]){
		root[a]=findroot(root[a]);
	}
	return root[a];
}

void unionroot(int root1,int root2){
	root[root1]=root2;
}

int main(){
	int T;
	scanf("%d",&T);
	for(int index=0;index<T;index++){
		int S,P;
		scanf("%d",&S);
		scanf("%d",&P);
		int k=0;
		int i=0;
		for(i=0;i<P;i++){
			int x,y;
			scanf("%d",&x);
			scanf("%d",&y);
			nodes[i].x=x;
			nodes[i].y=y;
		}
		int j=0;
		k=0;
		for(i=0;i<P;i++){
			for(j=0;j<i;j++){
				edges[k].from=i;
				edges[k].to=j;
				edges[k].cost=GetLength(nodes[i].x,nodes[i].y,nodes[j].x,nodes[j].y);
				k++;
			}
		}
		//init();
		qsort(edges,k,sizeof(Edge),cmp);
		int num=0;
		int kk=0;
		init();
		for(kk=0;kk<k;kk++){
			int a=edges[kk].from;
			int b=edges[kk].to;
			int root1=findroot(a);
			int root2=findroot(b);
			if(root1!=root2){
				unionroot(root1,root2);
				num++;
				if(num==P-S){
					printf("%.2f\n",edges[kk].cost);
					break;
				}
			}
		}
	}
	return 0;
}