【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[1..200000]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]:=1;
 inc(top); stack[top]:=u;
 inc(time); dfn[u]:=time; low[u]:=time;
 e:=head[u];
 while e<>0 do
 begin
  v:=vet[e];
  if flag[v]=0 then
  begin
   dfs(v);
   low[u]:=min(low[u],low[v]);
  end
   else if s[v]=0 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>0)and(stack[top]<>u) do
  begin
   s[stack[top]]:=id;
   inc(b[id]);
   stack[top]:=0;
   dec(top);
  end;
  stack[top]:=0;
  dec(top);
 end;
end;

begin
 //assign(input,'1.in'); reset(input);
 //assign(output,'1.out'); rewrite(output);
 readln(n);
 for i:=1 to n do
 begin
  read(x);
  add(i,x);
 end;
 for i:=1 to n do
  if flag[i]=0 then dfs(i);
 ans:=n;
 for i:=1 to id do
  if (b[i]>1)and(b[i]<ans) then ans:=b[i];
 if n=1 then writeln(0)
  else writeln(ans);
 //close(input);
 //close(output);
end.