tarjan

http://kmplayer.iteye.com/blog/604532

var        next,last,z,tov,low,dfn:array[1..1000000] of longint;        vis,zz:array[1..1000000] of boolean;        tot,n,m,i,x,y,time,zd,ans:longint;procedure insert(x,y:longint);begin        inc(tot);        tov[tot]:=y;        next[tot]:=last[x];        last[x]:=tot;end;function min(x,y:longint):longint;begin        if x<y then exit(x);        exit(y);end;procedure tarjan(x:longint);var        i,y,t:longint;begin        inc(time);        dfn[x]:=time;        low[x]:=time;        inc(zd);        z[zd]:=x;        zz[x]:=true;        i:=last[x];        while i<>0 do        begin                y:=tov[i];                if vis[y] then                begin                        vis[y]:=false;                        tarjan(y);                        low[x]:=min(low[x],low[y]);                end                else                if zz[y] then low[x]:=min(low[x],dfn[y]);                i:=next[i];        end;        if dfn[x]=low[x] then        begin                t:=0;                repeat                        zz[z[zd]]:=false;                        inc(t);                        dec(zd);                until z[zd+1]=x;                inc(ans,t*(t-1)div 2);        end;end;begin        readln(n,m);        for i:=1 to m do        begin                readln(x,y);                insert(x,y);        end;        fillchar(vis,sizeof(vis),true);        for i:=1 to n do                if vis[i] then                begin                        vis[i]:=false;                        tarjan(i);                end;                writeln(ans);end.