习题：疫情控制（二分+倍增+贪心）

program blockade;
type
  point=^node;
   node=record
     x,v:longint; next:point;
   end;
  arr=array[0..50000]of longint;
var
  w:array[0..50000]of point;
  f:array[0..30,0..50000]of longint;
  mark,flag:array[0..50000]of boolean;
  d,s,comefrom,army,a,b,rest,need,minr:array[0..50000]of longint;
  n,i,m,maxv,maxd,x,y,v:longint;
procedure qsort(l,h:longint; var a,b:arr);
var i,j,t,m:longint;
begin i:=l; j:=h;
 m:=a[(i+j) div 2];
 repeat
while a[i]>m do inc(i);
while m>a[j] do dec(j);
if i<=j then
  begin   t:=a[i]; a[i]:=a[j]; a[j]:=t; t:=b[i]; b[i]:=b[j]; b[j]:=t;
inc(i); dec(j);  end;  until i>j;
 if i<h then qsort(i,h,a,b); if j>l then qsort(l,j,a,b);  end;
procedure workfirst;
var i:longint;
begin
  fillchar(f,sizeof(f),0); fillchar(s,sizeof(s),0);
  maxv:=0; maxd:=0;
end;
procedure add(x,y,v:longint);
var p:point;
begin
  new(p); p^.x:=y;p^.v:=v; p^.next:=w[x];w[x]:=p;
end;
procedure dfs1(x,y,deep,v:longint);
var p:point; t:longint;
begin
  new(p); p:=w[x]; f[0,x]:=y; d[x]:=deep;  if x<>1 then comefrom[x]:=v;
  while p<>nil do
  begin
    if p^.x<>y then
     begin s[p^.x]:=s[x]+p^.v; if x=1 then t:=p^.x else t:=v; dfs1(p^.x,x,deep+1,t); end;
    p:=p^.next;
  end;
  if d[x]>maxd then maxd:=d[x];
end;
function dfs2(x:longint):boolean;
var p:point;
begin
  if mark[x]=true then exit(true);
  new(p); p:=w[x]; if (p^.x=f[0,x])and(p^.next=nil) then exit(false);
  while p<>nil do
   begin
     if p^.x<>f[0,x] then
     if dfs2(p^.x)=false then exit(false); p:=p^.next;
   end;
  exit(true);
end;
procedure find;
var i,j:longint;
begin
  for i:=0 to trunc(ln(maxd)/ln(2))-1 do
   for j:=0 to n do
     if f[i,j]<0 then f[i+1,j]:=-1
      else f[i+1,j]:=f[i,f[i,j]];
end;
function cheak(mid:longint):boolean;
var i,j,n1,n2,tot,t:longint; p:point;
begin
  n1:=0; n2:=0;
  fillchar(mark,sizeof(mark),false); fillchar(rest,sizeof(rest),0); rest[0]:=maxlongint;
  for i:=1 to m do
   begin
     x:=army[i]; tot:=0;
     for j:=trunc(ln(maxd)/ln(2)) downto 0 do
      begin
        y:=f[j,x];
        if y>0 then
        if (tot+s[x]-s[y]<=mid) then begin tot:=tot+s[x]-s[y]; x:=y; end;
        if x=1 then break;
      end;
     if x=1 then begin inc(n1); a[n1]:=i; rest[n1]:=mid-tot; end
     else mark[x]:=true;
   end;
  new(p);p:=w[1];
  while p<>nil do
   begin
     minr[p^.x]:=0;
     if dfs2(p^.x)=false then begin    inc(n2); b[n2]:=p^.x; need[n2]:=p^.v;  end; p:=p^.next;
   end;
  if n2=0 then exit(true);
  qsort(1,n1,rest,a); qsort(1,n2,need,b);
  for i:=1 to n1 do
   begin
     y:=comefrom[army[a[i]]]; if rest[i]<rest[minr[y]] then minr[y]:=i;
   end;
  i:=1;  fillchar(flag,sizeof(flag),true);
  for j:=1 to n2 do
   begin
      y:=b[j]; t:=minr[y];
     if (t>0)and(flag[t]=true) then begin flag[t]:=false; end
      else
        begin
          while (flag[i]=false)and(i<=n) do inc(i); if i>n then exit(false);
          if rest[i]<need[j] then exit(false) else begin flag[i]:=false; inc(a[i]); end;
        end;
   end;
  exit(true);
end;
procedure work;
var i,l,r,mid,ans:longint;
begin
  workfirst;
  dfs1(1,-1,1,0); for i:=1 to n do begin if s[i]>maxv then maxv:=s[i];  end;
  find;
  maxv:=maxv*2;
 l:=1; r:=maxv; ans:=r;
  while l<=r do
   begin
     mid:=(l+r) div 2;
     if cheak(mid)=true then begin ans:=mid; r:=mid-1; end
       else l:=mid+1;
   end;
  if l>maxv then ans:=-1;
  writeln(ans);
end;
begin
  readln(n);
  for i:=1 to n-1 do
   begin
     readln(x,y,v);
     add(x,y,v); add(y,x,v);
   end;
  readln(m);
  for i:=1 to m do
    read(army[i]);
  work;
end.