求樹的最大直徑

求樹直徑原理：以任意點w開始，先作一次DFS（BFS），找到最遠點v，而後以此點v，進行一次DFS（BFS），找到最遠點u，u到v就是樹的直徑，記作d(u,v)。

下邊兩道題堪稱樹直徑模板！

TOJ 3517 The longest athletic track

import java.util.*;

public class Main7 {
    static int Maxsize=100000;
    static int map[][]=new int[1001][1001];
    static int sum[]=new int[1001];
    static boolean vis[]=new boolean[1001];
    static int n,max;
    public static int bfs(int v,int n){
        
         for(int i=1;i<=n;i++)
         {     vis[i]=false;
             sum[i]=0;
         }
         max=0;
         Queue<Integer>q = new LinkedList<Integer>();
         q.offer(v);
         vis[v]=true;
         int count =0,k,key=0;
         while(!q.isEmpty()){
 
             k=q.poll(); 
                  
                 
              for(int i=1;i<=n;i++){ 
                 if(map[k][i]!=Maxsize&&vis[i]==false){
                     vis[i]=true;
                     q.offer(i);
 
                     sum[i]=sum[k]+map[k][i];
                     if(sum[i]>max){
                         max=sum[i];
                         key=i;
                     }
                 }
             }
//             System.out.println(max);
         }
         return key;
         
    }
     
    public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         int n = sc.nextInt();
         for(int i=1;i<=n;i++){
             for (int j=1; j<=n; j++)
                {
                    map[i][j]=Maxsize;
                }
                } 
                    
         for(int i=1;i<n;i++){
             int a= sc.nextInt();
             int b = sc.nextInt();
             int w = sc.nextInt();
              map[a][b]=map[b][a]=w;
         }
         int key;
            key=bfs(1,n);
//            System.out.println(max);
//            System.out.println("fsf");
            key=bfs(key,n);
            System.out.println(max);
    }

}

 

大臣的旅費（java）-藍橋杯

import java.util.*;

 
public class Main7 {
    static int Maxsize=100000;
    static int map[][]=new int[1001][1001];
//    static int sum[]=new int[1001];
    static boolean vis[]=new boolean[1001];
    static int s,n,max,sum;
    public static void dfs(int start,int n){
        if(sum>max){
            max=sum;
            s=start;
        }
        for(int i=1;i<=n;i++){
            if(!vis[i]&&map[start][i]!=Maxsize){
                vis[i]=true;
                sum+=map[start][i];
                dfs(i, n);
                sum-=map[start][i];
                vis[i]=false;
            }
        }
         return ;
         
    }
     
    public static void main(String[] args) {
         Scanner sc = new Scanner(System.in);
         int n = sc.nextInt();
         for(int i=1;i<=n;i++){
             for (int j=1; j<=n; j++)
                {
                    map[i][j]=Maxsize;
                }
                } 
         
         for(int i=1;i<n;i++){
             int a= sc.nextInt();
             int b = sc.nextInt();
             int w = sc.nextInt();
              map[a][b]=map[b][a]=w;
         }
         vis[1]=true;
         dfs(1, n);
         Arrays.fill(vis, false);
         vis[s]=true;
             dfs(s, n);
             int price=(10+max+11)*max/2;
             System.out.println(price);
    }

}