poj-2377-Bad Cowtractors--（最小生成树Kruskal）

Bessie has been hired to build a cheap internet network among Farmer John's N (2 <= N <= 1,000) barns that are conveniently numbered 1..N. FJ has already done some surveying, and found M (1 <= M <= 20,000) possible connection routes between pairs of barns. Each possible connection route has an associated cost C (1 <= C <= 100,000). Farmer John wants to spend the least amount on connecting the network; he doesn't even want to pay Bessie. 

Realizing Farmer John will not pay her, Bessie decides to do the worst job possible. She must decide on a set of connections to install so that (i) the total cost of these connections is as large as possible, (ii) all the barns are connected together (so that it is possible to reach any barn from any other barn via a path of installed connections), and (iii) so that there are no cycles among the connections (which Farmer John would easily be able to detect). Conditions (ii) and (iii) ensure that the final set of connections will look like a "tree".

Input

* Line 1: Two space-separated integers: N and M 

* Lines 2..M+1: Each line contains three space-separated integers A, B, and C that describe a connection route between barns A and B of cost C.

Output

* Line 1: A single integer, containing the price of the most expensive tree connecting all the barns. If it is not possible to connect all the barns, output -1.

Sample Input

5 8
1 2 3
1 3 7
2 3 10
2 4 4
2 5 8
3 4 6
3 5 2
4 5 17

Sample Output

42

Hint

OUTPUT DETAILS: 

The most expensive tree has cost 17 + 8 + 10 + 7 = 42. It uses the following connections: 4 to 5, 2 to 5, 2 to 3, and 1 to 3.

最小生成树问题，与题目poj-2395-Out of Hay-（Kruskal）差不多，只不过这个题是“最大生成树”，是往树中加入n-1个大的边，累加和就好了

代码：

#include<iostream>
#include<string>
#include<cstdio>
#include<algorithm>
#include<cmath>
#include<iomanip>
#include<queue>
#include<cstring>
#include<map>
using namespace std;
typedef long long ll;
#define pi acos(-1.0)
#define inf 0x3f3f3f
#define M 1005
struct node{
    int s,e,v;
}p[40005];
int n,m,fat[1005];
int father(int x)
{
    if(fat[x]!=x) fat[x]=father(fat[x]);
    return fat[x];
}
void unionn(int x,int y)
{
    int fa=father(x);
    int fb=father(y);
    if(fa!=fb) fat[fa]=fb;
}
bool comp(const node & a,const node & b)
{
    return a.v>b.v;
}
int main()
{
    int i,num;
    scanf("%d%d",&n,&m);
    for(i=1;i<=m;i++)
    {
        scanf("%d%d%d",&p[i].s,&p[i].e,&p[i].v);
        p[i+m].s=p[i].e;
        p[i+m].e=p[i].s;
        p[i+m].v=p[i].v;
    }
    m=m*2; num=0;
    ll sum=0;
    for(i=1;i<=n;i++) fat[i]=i;
    sort(p+1,p+m+1,comp);
    for(i=1;i<=m;i++)
    {
        if(father(p[i].s)!=father(p[i].e))
        {
            unionn(p[i].s,p[i].e);
            num++;
            sum+=p[i].v;
        }
        if(num==n-1) break;
    }
    if(num==n-1) printf("%I64d",sum);
    else printf("-1");    //num不能到达n-1，说明生成不了树
    return 0;
}