1486: [HNOI2009]最小圈 – BZOJ

 在机房的小伙伴提醒是二分之后，我想到了是判负环，所以我用spfa，而且我保持dis都是小于等于0，本以为这样就能过了，可是还是有一个点达到了3.8s左右（其他都是0.0几秒）所以还是写了dfs版本，还是一样每次都保持dis小于等于0，当发现有一个点在栈中，你又可以更新他的dis，那么就有负环了

 const
     maxn=;
     maxm=;
     inf=;
 var
     first:array[..maxn]of longint;
     next,last:array[..maxm]of longint;
     w:array[..maxm]of double;
     n,m:longint; procedure init;
 var
     i,x:longint;
 begin
     read(n,m);
     for i:= to m do
       begin
         read(x);
         read(last[i],w[i]);
         next[i]:=first[x];
         first[x]:=i;
       end;
 end; var
     flag,vis:array[..maxn]of longint;
     dis:array[..maxn]of double;
     time,time2:longint;
     ans:double; function dfs(x:longint;f:double):boolean;
 var
     i:longint;
 begin
     flag[x]:=time;
     vis[x]:=time2;
     dis[x]:=f;
     i:=first[x];
     while i<> do
       begin
         if f+w[i]-ans<= then
         begin
           if vis[last[i]]=time2 then
             if dis[last[i]]>=f+w[i]-ans then exit(true);
           if vis[last[i]]<>time2 then
             if dfs(last[i],f+w[i]-ans) then exit(true);
         end;
         i:=next[i];
       end;
     vis[x]:=time2-;
     exit(false);
 end; procedure work;
 var
     i:longint;
     l,r:double;
     f:boolean;
 begin
     l:=-;
     r:=-l;
     while r-l>1e-9 do
       begin
         ans:=(l+r)/;
         inc(time);
         f:=false;
         for i:= to n do
           if flag[i]<>time then
           begin
             inc(time2);
             if dfs(i,) then
             begin
               f:=true;
               break;
             end;
           end;
         if f then r:=ans
         else l:=ans;
       end;
     writeln(ans::);
 end; begin
     init;
     work;
 end.