Build The Electric System
Time Limit: 2 Seconds      
Memory Limit: 65536 KB

In last winter, there was a big snow storm in South China. The electric system was damaged seriously. Lots of power lines were broken and lots of villages lost contact with the main power grid. The government wants to reconstruct the electric system as soon as possible. So, as a professional programmer, you are asked to write a program to calculate the minimum cost to reconstruct the power lines to make sure there’s at least one way between every two villages.

Input

Standard input will contain multiple test cases. The first line of the input is a single integer T (1 <= T <= 50) which is the number of test cases. And it will be followed by T consecutive test cases.

In each test case, the first line contains two positive integers N and E (2 <= N <= 500, N <= E <= N * (N – 1) / 2), representing the number of the villages and the number of the original power lines between villages. There follow E lines, and each of them contains three integers, A, B, K (0 <= A, B < N, 0 <= K < 1000). A and B respectively means the index of the starting village and ending village of the power line. If K is 0, it means this line still works fine after the snow storm. If K is a positive integer, it means this line will cost K to reconstruct. There will be at most one line between any two villages, and there will not be any line from one village to itself.

Output

For each test case in the input, there’s only one line that contains the minimum cost to recover the electric system to make sure that there’s at least one way between every two villages.

Sample Input

1
3 3
0 1 5
0 2 0
1 2 9

Sample Output

5

题意：大概是说将所有村庄连接起来最小的花费

题解：最小生成树裸题。

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
#include<vector>
using namespace std;
#define LL long long 
#define N 10000
int fa[N];
#define inf 0x3f3f3f3f
struct point
{
	int x,y,v;;
	point (int _x,int _y,int _v):x(_x),y(_y),v(_v){}
	point(){}
	bool operator< (const point &x1) const
	{
		return this->v<x1.v;
	}
};
vector<point>eg;
int find(int x)
{
	return x==fa[x]?x:fa[x]=find(fa[x]);
}
void unio(int x,int y)
{
	x=find(x);
	y=find(y);
	if(fa[x]!=y)
		fa[x]=y;
}

int kruskal()
{
	int ans=0;
	for(int i=0;i<eg.size();i++)
	{
		if(find(eg[i].x)!=find(eg[i].y))
		{
			unio(eg[i].x,eg[i].y);
			ans+=eg[i].v;
		}
	}
	return ans;
}
int main(){
#ifdef CDZSC
	freopen("i.txt","r",stdin);
#endif
	char s[N];
	int n,t,m,x,y,va;
	scanf("%d",&t);
	while(t--)
	{
		eg.clear();
		scanf("%d%d",&n,&m);
		for(int i=0;i<n;i++)
			fa[i]=i;
		for(int i=0;i<m;i++)
		{
			scanf("%d%d%d",&x,&y,&va);
			eg.push_back(point(x,y,va));
		}
		sort(eg.begin(),eg.end());
		printf("%d\n",kruskal());
	}
	return 0; 
}