bzoj 1776

收获：

树上直径一定包含深度最深的点。

然后O(nlogn) 暴力。

 /**************************************************************
     Problem: 1776
     User: idy002
     Language: C++
     Result: Accepted
     Time:2064 ms
     Memory:31756 kb
 ****************************************************************/

 #include <cstdio>
 #include <vector>
 #define maxn 200010
 #define maxp 18
 using namespace std;

 int n, m, root;
 int anc[maxn][maxp+], dep[maxn];
 vector<int> g[maxn];
 vector<int> grp[maxn];

 void dfs( int u ) {
     for( int p=; p<=maxp; p++ )
         anc[u][p]=anc[anc[u][p-]][p-];
     for( int t=; t<g[u].size(); t++ ) {
         int v=g[u][t];
         anc[v][] = u;
         dep[v] = dep[u]+;
         dfs( v );
     }
 }
 int lca( int u, int v ) {
     if( dep[u]<dep[v] ) swap(u,v);
     int t=dep[u]-dep[v];
     for( int p=; t; t>>=,p++ )
         if( t& ) u=anc[u][p];
     if( u==v ) return u;
     for( int p=maxp; anc[u][]!=anc[v][]; p-- )
         if( anc[u][p]!=anc[v][p] )
             u=anc[u][p],v=anc[v][p];
     return anc[u][];
 }
 int main() {
     scanf( "%d%d", &n, &m );
     for( int i=,a,b; i<=n; i++ ) {
         scanf( "%d%d", &a, &b );
         grp[a].push_back( i );
         if( b== ) root=i;
         else g[b].push_back( i );
     }
     dep[root] = ;
     anc[root][] = root;
     dfs(root);
     for( int i=; i<=m; i++ ) {
         int md = grp[i][];
         for( int t=; t<grp[i].size(); t++ )
             if( dep[grp[i][t]]>dep[md] )
                 md=grp[i][t];
         int u=md;
         int ans=;
         for( int t=; t<grp[i].size(); t++ ) {
             int v=grp[i][t];
             int ca=lca(u,v);
             int dis = dep[u]+dep[v]-dep[ca]-dep[ca];
             if( ans<dis ) ans=dis;
         }
         printf( "%d\n", ans );
     }
 }