3140: [Hnoi2013]消毒

首先要知道一次打掉一个1*x*y的面是最优的。证明太简单所以略。

然后看这道题的二维版本：bzoj1693 [Usaco2007 Demo]Asteroids   http://www.lydsy.com/JudgeOnline/problem.php?id=1693

然后这样的题一般都是一维暴力来减少一维。就枚举最小的一维打掉哪些面。然后把剩下的压成两维，然后做。

RunID	User	Problem	Result	Memory	Time	Language	Code_Length	Submit_Time
410333	lbz007	1693	Accepted	12032 kb	4 ms	Pascal/Edit	862 B	2013-05-11 20:51:11

var
  u,v,k,ans,i,j,n,a,ee:longint;
  link:Array[0..2002]of longint;
  next,head,e:array[0..1000000]of longint;
  b:array[0..20002]of longint;
procedure add(u,v:longint);
  begin
  inc(ee);
  next[ee]:=head[u];
  head[u]:=ee;
  e[ee]:=v;
  end;
  
function find(x:longint):boolean;
  var j:longint;
  begin
  j:=head[x];
  while j<>0 do
    begin
    if b[e[j]]<>i then
      begin
      b[e[j]]:=i;
      if (link[e[j]]=0) or (find(link[e[j]])) then
        begin
        link[e[j]]:=x;
        exit(true);
        end;
      end;
    j:=next[j];
    end;
  exit(false);
end;
begin
  readln(n,k);
  for i:=1 to k do
    begin
    readln(u,v);
    add(u,v);
    end;
  for i:=1 to n do
    if find(i) then inc(ans);
  writeln (ans);
end.