【CF500D】New Year Santa Network（树上统计）

var head,vet,next,x,y,dep,size:array[1..200000]of longint;
    z:array[1..200000]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]:=1;
 e:=head[u];
 while e<>0 do
 begin
  v:=vet[e];
  if v<>fa then
  begin
   dep[v]:=dep[u]+1;
   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-1)/2*yy+yy*(yy-1)/2*xx));
end;

begin
 //assign(input,'1.in'); reset(input);
 //assign(output,'1.out'); rewrite(output);
 readln(n);
 for i:=1 to n-1 do
 begin
  read(x[i],y[i],z[i]);
  add(x[i],y[i]);
  add(y[i],x[i]);
 end;
 dfs(1,0);
 for i:=1 to n-1 do
  if dep[x[i]]>dep[y[i]] then swap(x[i],y[i]);
 fenmu:=1.0*n*(n-1)*(n-2)/6;
 for i:=1 to n-1 do ans:=ans+clac(i);


 readln(m);
 for i:=1 to m do
 begin
  readln(x1,y1);
  ans:=ans-clac(x1);
  z[x1]:=y1;
  ans:=ans+clac(x1);
  writeln(ans:0:10);
 end;

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