题目连接：http://acm.hdu.edu.cn/showproblem.php?pid=1102

Constructing Roads

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 65536/32768 K (Java/Others)
Total Submission(s): 21947    Accepted Submission(s):
8448

Problem Description

There are N villages, which are numbered from 1 to N,
and you should build some roads such that every two villages can connect to each
other. We say two village A and B are connected, if and only if there is a road
between A and B, or there exists a village C such that there is a road between A
and C, and C and B are connected.

We know that there are already some
roads between some villages and your job is the build some roads such that all
the villages are connect and the length of all the roads built is
minimum.

Input

The first line is an integer N (3 <= N <= 100),
which is the number of villages. Then come N lines, the i-th of which contains N
integers, and the j-th of these N integers is the distance (the distance should
be an integer within [1, 1000]) between village i and village j.

Then
there is an integer Q (0 <= Q <= N * (N + 1) / 2). Then come Q lines, each
line contains two integers a and b (1 <= a < b <= N), which means the
road between village a and village b has been built.

Output

You should output a line contains an integer, which is
the length of all the roads to be built such that all the villages are
connected, and this value is minimum.

Sample Input

3

0 990 692

990 0 179

692 179 0

1

1 2

Sample Output

179

题目大意：有N个村庄，现在需要修一些路，使得任意两个村庄之间可以连通（可以间接相连），给出每条路的花费，求出最少需要的花费。已知一些路已经修好了。

解题思路：根据题目的输入数据，可以选用以顶点为主导的Prim算法求MST。

AC代码：

 1 #include <stdio.h>
 2 #include <string.h>
 3 #define inf 0x3f3f3f3f
 4
 5 int map[110][110],vis[110],dis[110];
 6 int n;
 7
 8 void prim()
 9 {
10     int i,j,pos,min,sum = 0;
11     for (i = 1; i <= n; i ++)
12     {
13         vis[i] = 0;
14         dis[i] = map[1][i];
15     }
16     vis[1] = 1; dis[1] = 0;
17     for (i = 1; i < n; i ++)
18     {
19         pos = -1; min = inf;
20         for (j = 1; j <= n; j ++)
21         {
22             if (!vis[j] && min > dis[j])
23             {
24                 min = dis[j];
25                 pos = j;
26             }
27         }
28         vis[pos] = 1;
29         sum += min;
30         for (j = 1; j <= n; j ++)
31             if (!vis[j] && dis[j] > map[pos][j])
32                 dis[j] = map[pos][j];
33     }
34     printf("%d\n",sum);
35 }
36 int main ()
37 {
38     int i,j,m,a,b;
39     while (~scanf("%d",&n))
40     {
41         for (i = 1; i <= n; i ++)
42         for (j = 1; j <= n; j ++)
43             scanf("%d",&map[i][j]);
44         scanf("%d",&m);
45         for (i = 0; i < m; i ++)
46         {
47             scanf("%d%d",&a,&b);
48             map[a][b] = map[b][a] = 0;
49         }
50         prim();
51     }
52     return 0;
53 }

算法理解： http://www.cnblogs.com/yoke/p/6506492.html