【NOIP2015】信息传递（强连通分量）

题意：找一张图中的最小环

O(n)

思路：强连通分量tarjan即可 注意环中节点数>1

 var head,vet,next,s,b,stack,low,dfn,flag:array[..]of longint;
     n,i,ans,tot,id,top,time,x:longint;

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

 function min(x,y:longint):longint;
 begin
  if x<y then exit(x);
  exit(y);
 end;

 procedure dfs(u:longint);
 var e,v:longint;
 begin
  flag[u]:=;
  inc(top); stack[top]:=u;
  inc(time); dfn[u]:=time; low[u]:=time;
  e:=head[u];
  while e<> do
  begin
   v:=vet[e];
   if flag[v]= then
   begin
    dfs(v);
    low[u]:=min(low[u],low[v]);
   end
    else if s[v]= then low[u]:=min(low[u],low[v]);
   e:=next[e];
  end;

  if low[u]=dfn[u] then
  begin
   inc(id); s[u]:=id; inc(b[id]);
   while (top>)and(stack[top]<>u) do
   begin
    s[stack[top]]:=id;
    inc(b[id]);
    stack[top]:=;
    dec(top);
   end;
   stack[top]:=;
   dec(top);
  end;
 end;

 begin
  //assign(input,'1.in'); reset(input);
  //assign(output,'1.out'); rewrite(output);
  readln(n);
  for i:= to n do
  begin
   read(x);
   add(i,x);
  end;
  for i:= to n do
   if flag[i]= then dfs(i);
  ans:=n;
  for i:= to id do
   if (b[i]>)and(b[i]<ans) then ans:=b[i];
  if n= then writeln()
   else writeln(ans);
  //close(input);
  //close(output);
 end.