【CF500D】New Year Santa Network（树上统计）

 var head,vet,next,x,y,dep,size:array[..]of longint;
     z:array[..]of extended;
     n,i,m,tot,x1:longint;
     ans,fenmu,y1:extended;

 procedure add(a,b:longint);
 begin
  inc(tot);
  next[tot]:=head[a];
  vet[tot]:=b;
  head[a]:=tot;
 end;

 procedure dfs(u,fa:longint);
 var e,v:longint;
 begin
  size[u]:=;
  e:=head[u];
  while e<> do
  begin
   v:=vet[e];
   if v<>fa then
   begin
    dep[v]:=dep[u]+;
    dfs(v,u);
    size[u]:=size[u]+size[v];
   end;
   e:=next[e];
  end;
 end;

 procedure swap(var x,y:longint);
 var t:longint;
 begin
  t:=x; x:=y; y:=t;
 end;

 function clac(k:longint):extended;
 var xx,yy:extended;
 begin
  xx:=size[y[k]];
  yy:=n-xx;
  exit(2.0*z[k]/fenmu*(xx*(xx-)/*yy+yy*(yy-)/*xx));
 end;

 begin
  //assign(input,'1.in'); reset(input);
  //assign(output,'1.out'); rewrite(output);
  readln(n);
  for i:= to n- do
  begin
   read(x[i],y[i],z[i]);
   add(x[i],y[i]);
   add(y[i],x[i]);
  end;
  dfs(,);
  for i:= to n- do
   if dep[x[i]]>dep[y[i]] then swap(x[i],y[i]);
  fenmu:=1.0*n*(n-)*(n-)/;
  for i:= to n- do ans:=ans+clac(i);

  readln(m);
  for i:= to m do
  begin
   readln(x1,y1);
   ans:=ans-clac(x1);
   z[x1]:=y1;
   ans:=ans+clac(x1);
   writeln(ans::);
  end;

  //close(input);
  //close(output);
 end.