问题描述：
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
问题分析：
将每条按费用高低从低到高排列起来 ，再用Kruskal算法选择最小总费用，实际就是利用并查集集成一棵树，因为费用已经从小排到大了，选全部最小的路做为一颗树，再选第二高的，若可以与第一棵树连则连起来，不能则另起一颗（即并查集），因为都是从最小的选起来，则生成的第一棵完整的树即是最小总费用的树。
ac代码：
#include
#include
#include
#include
using namespace std;
#define max 1500
int pa[55];
int ans, m;
struct asd
{
int a, s, d;
}p[max];
bool cmp(asd f, asd b)
{
return f.d < b.d;
}
int find(int x)
{
while (x != pa[x])
x = pa[x];
return x;
}
void bcj()
{
sort(p, p + m, cmp);
for (int j = 0; j < m; j++)
{
int x = find(p[j].a), y = find(p[j].s);
if (x != y)
{
ans += p[j].d;
pa[x] = y;
}
}
}
int main()
{
int n;
while (cin >> n)
{
if (n == 0)break;
cin >> m;
for (int i = 0; i < m; i++)
cin >> p[i].a >> p[i].s>> p[i].d;
for (int i = 1; i <= 54; i++)
pa[i] = i;
ans = 0;
bcj();
cout << ans << endl;
}
}