HDU 4607 Park Visit 两次DFS求树直径

两次DFS求树直径方法见 这里。

这里的直径是指最长链包含的节点个数，而上一题是指最长链的路径权值之和，注意区分。

K <= R: ans = K − 1;

K > R:   ans = R − 1 + ( K − R ) ∗ 2;

 #include <cstdio>
 #include <cstring>
 #include <cstdlib>
 #include <algorithm>

 using namespace std;

 const int MAXN = ;

 struct node
 {
     int v;
     int next;
 };

 int N, Q, EdgeN;
 int head[MAXN];
 int best[MAXN];
 int dp[MAXN][];
 node D[ MAXN <<  ];
 bool vis[MAXN];

 void AddEdge( int u, int v )
 {
     D[EdgeN].v = v;
     D[EdgeN].next = head[u];
     head[u] = EdgeN++;
     return;
 }

 void DFS1( int u )
 {
     vis[u] = true;
     for ( int i = head[u]; i != -; i = D[i].next )
     {
         int v = D[i].v;
         int w = ;
         if ( !vis[v] )
         {
             DFS1( v );
             if ( dp[v][] + w > dp[u][] )
             {
                 dp[u][] = dp[u][];
                 dp[u][] = dp[v][] + w;
                 best[u] = v;
             }
             else if ( dp[v][] + w > dp[u][] )
                 dp[u][] = dp[v][] + w;
         }
     }
     return;
 }

 void DFS2( int u )
 {
     vis[u] = true;
     for ( int i = head[u]; i != -; i = D[i].next )
     {
         int fa = D[i].v;
         int w = ;
         if ( !vis[fa] )
         {
             dp[fa][] = dp[u][] + w;
         if ( fa == best[u] )
             dp[fa][] = max( dp[fa][], dp[u][] + w );
         else dp[fa][] = max( dp[fa][], dp[u][] + w );
         DFS2( fa );
         }
     }
     return;
 }

 int main()
 {
     int T;
     scanf( "%d", &T );
     while ( T-- )
     {
         scanf( "%d%d", &N, &Q );
         EdgeN = ;
         memset( head, -, sizeof(head) );
         for ( int i = ; i < N; ++i )
         {
             int u, v;
             scanf( "%d%d", &u, &v );
             AddEdge( u, v );
             AddEdge( v, u );
         }

         memset( dp, , sizeof(dp) );
         memset( vis, false, sizeof(bool) * (N + ) );
         DFS1(  );
         memset( vis, false, sizeof(bool) * (N + ) );
         DFS2(  );

         int maxx = ;
         for ( int i = ; i <= N; ++i )
             maxx = max( maxx, max( dp[i][], dp[i][] ) );
         ++maxx;

         while ( Q-- )
         {
             int K;
             scanf( "%d", &K );
             if ( maxx >= K )
                 printf( "%d\n", K -  );
             else printf( "%d\n", maxx -  + ( K - maxx ) *  );
         }
     }
     return ;
 }