noip2014 寻找道路

const   maxn=10010;    maxm=200010;var nextt,too:array[0..2*maxm] of longint;    x,y:array[0..maxm] of longint;    head,q,step:array[0..maxn] of longint;    vis:array[0..maxn] of boolean;    n,m,s,t,i,tot:longint;procedure add(a,b:longint);begin    inc(tot);    nextt[tot]:=head[a];    too[tot]:=b;    head[a]:=tot;end;function f(u:longint):boolean;var i:longint;begin    i:=head[u];    while i>0 do begin        if not vis[too[i]] then exit(false);        i:=nextt[i];    end;    exit(true);end;function bfs2:boolean;var l,r,u,i:longint;begin    q[0]:=s;    step[s]:=0;    l:=0;    r:=1;    while l<r do begin        u:=q[l];        inc(l);        if not f(u) then continue;        i:=head[u];        while i>0 do begin            if step[too[i]]=-1 then begin                step[too[i]]:=step[u]+1;                q[r]:=too[i];                inc(r);                if too[i]=t then begin                    writeln(step[too[i]]);                    exit(true);                end;             end;            i:=nextt[i];        end;    end;    exit(false);end;procedure bfs1;var l,r,u,i:longint;begin    q[0]:=t;    vis[t]:=true;    l:=0;    r:=1;    while l<r do begin        u:=q[l];        inc(l);        i:=head[u];        while i>0 do begin             if not vis[too[i]] then begin                 vis[too[i]]:=true;                 q[r]:=too[i];                inc(r);            end;            i:=nextt[i];        end;    end;end;begin    tot:=0;    fillchar(vis,sizeof(vis),false);    fillchar(nextt,sizeof(nextt),0);    fillchar(head,sizeof(head),0);    fillchar(too,sizeof(too),0);    readln(n,m);    for i:=1 to m do begin        readln(x[i],y[i]);        add(y[i],x[i]);    end;    readln(s,t);    bfs1;    fillchar(nextt,sizeof(nextt),0);    fillchar(head,sizeof(head),0);    fillchar(too,sizeof(too),0);    for i:=0 to maxn do step[i]:=-1;    fillchar(q,sizeof(q),0);    tot:=0;    for i:=1 to m do add(x[i],y[i]);    if not vis[s] then writeln(-1)        else if not bfs2 then writeln(-1);end.