【距离GDKOI：44天&GDOI：107天】【BZOJ1040】[ZJOI2008] 骑士 (环套树DP)

其实已经准备退役了，但GDOI之前还是会继续学下去的！！当成兴趣在学，已经对竞赛失去信心了的样子，我还是回去跪跪文化课吧QAQ

第一道环套树DP...其实思想挺简单的，就把环拆开，分类处理。若拆成开的两个点是u,v，dp[i,0..1]分别表示第i位骑士不选和选

(1) 不选u，v点随意    (2)u随意，v点不选...

分类dp处理即可

 const maxn=;
 type
  edgetype=record
   toward,next:longint;
  end;

 var
  edge:array[..maxn*] of edgetype;
  first:array[..maxn] of longint;
  val:array[..maxn] of int64;
  dp:array[..maxn,..] of int64;
  pd,vb,vc:array[..maxn] of boolean;
  root,vv,e,tot:longint;
  n:longint;
 function max(x,y:int64):int64; begin if x>y then exit(x) else exit(y); end;

 procedure addedge(i,j:longint);
 begin
  edge[tot].toward:=j;
  edge[tot].next:=first[i];
  first[i]:=tot;
  inc(tot);
 end;

 procedure add(i,j:longint);
 begin
  addedge(i,j); addedge(j,i);
 end;

 procedure dfs(v,fa:longint);
 var i,tmp:longint;
 begin
  pd[v]:=true;
  i:=first[v];
  while i<>- do
   begin
    tmp:=edge[i].toward;
    if not pd[tmp] then dfs(tmp,v)
     else if tmp<>fa then
      begin
       vv:=v;
       root:=tmp;
       e:=i;
      end;
    i:=edge[i].next;
   end;
 end;

 procedure dpb(v:longint); //ban u
 var i,tmp:longint;
 begin
  dp[v,]:=; dp[v,]:=val[v]; vb[v]:=true;
  i:=first[v];
  while i<>- do
   begin
    tmp:=edge[i].toward;
    if (i<>e) and (i xor <>e) and not vb[tmp] then
     begin
      dpb(tmp);
      dp[v,]:=dp[v,]+max(dp[tmp,],dp[tmp,]);
      dp[v,]:=dp[v,]+dp[tmp,];
     end;
    i:=edge[i].next;
   end;
 end;

 procedure dpc(v:longint); //ban v;
 var i,tmp:longint;
 begin
  dp[v,]:=; dp[v,]:=val[v]; vc[v]:=true;
  i:=first[v];
  while i<>- do
   begin
    tmp:=edge[i].toward;
    if (i<>e) and (i xor  <>e) and not vc[tmp] then
     begin
      dpc(tmp);
      dp[v,]:=dp[v,]+dp[tmp,];
      if tmp=vv then dp[v,]:=dp[v,]+dp[tmp,]
       else dp[v,]:=dp[v,]+max(dp[tmp,],dp[tmp,]);
     end;
    i:=edge[i].next;
   end;
 end;

 procedure solve;
 var i:longint;
  rec,ans:int64;
 begin
  ans:=;
  for i:=  to n do
   if not pd[i] then
    begin
     rec:=; root:=-; dfs(i,-);
     dpb(root); rec:=dp[root,];
     dpc(root); rec:=max(rec,max(dp[root,],dp[root,]));
     inc(ans,rec);
    end;
  writeln(ans);
 end;

 procedure init;
 var i,x:longint;
 begin
  fillchar(first,sizeof(first),);
  readln(n);
  tot:=;
  for i:=  to n do
   begin
    readln(val[i],x);
    add(i,x);
   end;
 end;

 Begin
  init;
  solve;
 End.