POJ 1287 Networking (最小生成树模板题）

Description

You are assigned to design network connections between certain points in a wide area. You are given a set of points in the area, and a set of possible routes for the cables that may connect pairs of points. For each possible route between two points, you are given the length of the cable that is needed to connect the points over that route. Note that there may exist many possible routes between two given points. It is assumed that the given possible routes connect (directly or indirectly) each two points in the area. 
Your task is to design the network for the area, so that there is a connection (direct or indirect) between every two points (i.e., all the points are interconnected, but not necessarily by a direct cable), and that the total length of the used cable is minimal.

Input

The input file consists of a number of data sets. Each data set defines one required network. The first line of the set contains two integers: the first defines the number P of the given points, and the second the number R of given routes between the points. The following R lines define the given routes between the points, each giving three integer numbers: the first two numbers identify the points, and the third gives the length of the route. The numbers are separated with white spaces. A data set giving only one number P=0 denotes the end of the input. The data sets are separated with an empty line. 
The maximal number of points is 50. The maximal length of a given route is 100. The number of possible routes is unlimited. The nodes are identified with integers between 1 and P (inclusive). The routes between two points i and j may be given as i j or as j i. 

Output

For each data set, print one number on a separate line that gives the total length of the cable used for the entire designed network.

Sample Input

1 0

2 3
1 2 37
2 1 17
1 2 68

3 7
1 2 19
2 3 11
3 1 7
1 3 5
2 3 89
3 1 91
1 2 32

5 7
1 2 5
2 3 7
2 4 8
4 5 11
3 5 10
1 5 6
4 2 12

0

Sample Output

0
17
16
26

求最小生成树基本思想

1. 定义结构体保存两节点及其距离
2. 对结构体排序（按两节点距离从小到大）
3. 对边的数量进行查询，若两节点父节点不同则连接两父节点，记录边的大小sum及有效边的数量k
4. 在循环中判断有效边数量，若等于节点数减一则结束循环
5. 判断有效边数量若等于节点数减一，则能连接所有节点输出值，否则不能

#include<cstdio>
#include<algorithm>
using namespace std;
int n,m,fa[100000],i,sum,k;

struct stu
{
    int from,to,al;
}st[1000000];

bool cmp(stu a,stu b)
{
    return a.al < b.al;
}

int find(int a)
{
    int r=a;
    while(r!=fa[r])
    {
        r=fa[r];
    }
    return r;
}

void init()
{
    for(i = 1 ; i <= n ;i++)
    {
        fa[i]=i;
    }
}

int judge(int x,int y)
{
    int xx=find(x);
    int yy=find(y);
    if(xx != yy)
    {
        fa[xx]=yy;
        return 1;
    }
    return 0;
}


int main()
{
    while(scanf("%d",&n) && n)
    {
        init();
        scanf("%d",&m);
        for(i = 0 ; i < m ; i++)
        {
            scanf("%d %d %d",&st[i].from,&st[i].to,&st[i].al); //定义结构体保存两节点及其距离
        }
        sort(st,st+m,cmp);    //对结构体排序（按两节点距离从小到大）
        int k = 0;
        int sum=0;
        for(i = 0 ; k < n-1 ; i++)        //对边的数量进行查询
        {
            if(judge(st[i].from,st[i].to))    //若两节点父节点不同则连接两父节点
            {
                k++;                        //
                sum+=st[i].al;
            }                                //在循环中判断有效边数量，若等于节点数减一则结束循环(写在循环里看k<ｎ－１ ） 
        }
        printf("%d
",sum);
    }
}