POJ - 1251A - Jungle Roads 利用最小生成树

The Head Elder of the tropical island of Lagrishan has a problem. A burst of foreign aid money was spent on extra roads between villages some years ago. But the jungle overtakes roads relentlessly, so the large road network is too expensive to maintain. The Council of Elders must choose to stop maintaining some roads. The map above on the left shows all the roads in use now and the cost in aacms per month to maintain them. Of course there needs to be some way to get between all the villages on maintained roads, even if the route is not as short as before. The Chief Elder would like to tell the Council of Elders what would be the smallest amount they could spend in aacms per month to maintain roads that would connect all the villages. The villages are labeled A through I in the maps above. The map on the right shows the roads that could be maintained most cheaply, for 216 aacms per month. Your task is to write a program that will solve such problems.

Input

The input consists of one to 100 data sets, followed by a final line containing only 0. Each data set starts with a line containing only a number n, which is the number of villages, 1 < n < 27, and the villages are labeled with the first n letters of the alphabet, capitalized. Each data set is completed with n-1 lines that start with village labels in alphabetical order. There is no line for the last village. Each line for a village starts with the village label followed by a number, k, of roads from this village to villages with labels later in the alphabet. If k is greater than 0, the line continues with data for each of the k roads. The data for each road is the village label for the other end of the road followed by the monthly maintenance cost in aacms for the road. Maintenance costs will be positive integers less than 100. All data fields in the row are separated by single blanks. The road network will always allow travel between all the villages. The network will never have more than 75 roads. No village will have more than 15 roads going to other villages (before or after in the alphabet). In the sample input below, the first data set goes with the map above.

Output

The output is one integer per line for each data set: the minimum cost in aacms per month to maintain a road system that connect all the villages. Caution: A brute force solution that examines every possible set of roads will not finish within the one minute time limit.

Sample Input

9
A 2 B 12 I 25
B 3 C 10 H 40 I 8
C 2 D 18 G 55
D 1 E 44
E 2 F 60 G 38
F 0
G 1 H 35
H 1 I 35
3
A 2 B 10 C 40
B 1 C 20
0

Sample Output

216
30
解题思路：

大致题意：在相通n个岛屿的所有桥都坏了，要重修，重修每一个桥所用的时间不同，求重修使每个岛屿都间接或直接与其他岛屿相连时所用的的最短时间（只有修完一个桥后才可修下一个桥）。
简言之就是求最小生成树。

对于数据，数据输入的第一行n代表岛屿的个数，当为0是结束程序，接着n-1行开始时为这岛屿的编号，用大写字母表示，接着是一个整数m
代码如下：

 #include<iostream>
 #include<stdio.h>
 #include<algorithm>
 using namespace std;

 int n , m ,k;
 char a , b ;
 struct edge{
     int u ;
     int v ;
     int w ;
 }e[];
 int pre[];
 bool cmp(edge a ,edge b)
 {
     return a.w < b.w;
 }
 int find(int x)
 {
     return (x==pre[x])?x:pre[x] = find(pre[x]);
 }
 int main()
 {
     while(scanf("%d",&n)!=EOF)
     {
         if(n==)
         break;
         int edge_num = ;
         for(int i =  ; i <=  ;i++)
         {
             pre[i] = i;
         }

         for(int i =  ; i < n ;i++)
         {
             cin>>a>>m;
             for(int j =  ; j < m ; j++)
             {
                 cin>>b>>k;
                 e[edge_num].u = a ;
                 e[edge_num].v = b ;
                  e[edge_num].w = k;
                  edge_num++;
             }
         }
         int ans =  ;
         sort(e,e+edge_num,cmp);
         for(int i =  ; i < edge_num ;i++)
         {
             int u = e[i].u;
             int v = e[i].v;
             int w = e[i].w;
             int fx = find(u);
             int fy = find(v);
             if(fx!=fy)
             {
                 pre[fx] = fy;
                 ans += w;
             }else
             {
                 continue;
             }
         }
         printf("%d\n",ans);
     }
     return ;
 }