最小生成树（Prim）

思路：从当前的某一点出发，每次选择当前能到达最小值的点，然后标记访问过的点和更新能到达的点，重复执行直至所有点都被访问过。

代码：

#include<iostream>

using namespace std;

const int maxn = 1000+10;

const int INF = 0x3fffffff;                               //  无穷的一半

int map[maxn][maxn];                                      //存图

int dis[maxn];

int visited[maxn];

int m,n;

void init(){                                             //map的初始化

         for(int i=1; i<=n; i++)

                   for(int j=1; j<=m; j++)

                            map[i][j] = INF;

}

int prim(int m){

         int t,min,ans=0;

         int i, j;

         memset(visited,0,sizeof(visited));                    //标记函数的初始化，初始值为0.

         for(i=1; i<=m; i++){

                   dis[i] = map[1][i];

         }

         visited[1] = 1;                                      //标记使用过

         for(i=1; i<m; i++){

                   min = INF;

                   t = -1;

                   for(j=1; j<=m; j++){

                            if(!visited[j] && dis[j] < min){

                                     min = dis[j];

                                     t = j;

                            }

                   }

                   if(t == -1)                                    //没有最少生成树时直接返回

                            return INF;

                   ans += min;

                   visited[t] = 1;

                   for(int k=1; k<=m; k++){

                            if(!visited[k] && dis[k] > map[t][k]){     //没有使用过并且比当前更优时替换

                                     dis[k] = map[t][k];

                            }

                   }

         }

         return ans;

}

int main(){

         int a, b, t;

         while(cin>>n>>m, n){                        //   n为边的条数，m 为点的个数

                   init();

                   while(n--){

                            cin>>a>>b>>t;

                            map[a][b] = map[b][a] = t;

                   }

                   t = prim(m);

                   if(t != INF)

                            cout<<t<<endl;

                   else

                            cout<<"没有最小生成树"<<endl;

         }

         return 0;

}