NKONEARC - Mạng máy tính
Tác giả: flashmt
Ngôn ngữ: Pascal
const fi='';
fo='';
maxn=2005;
maxm=30005;
var n,sm,m,dem,last,re,ri,rj,ne,nf:longint;
a:array[1..maxn,1..maxn] of boolean;
b:array[1..maxm,0..1] of longint;
free,e,f:array[1..maxn] of boolean;
low,num,d,st:array[1..maxn] of longint;
procedure rf;
var i,x,y:longint;
begin
assign(input,fi); reset(input);
readln(n,m);
for i:=1 to m do
begin
readln(b[i,0],b[i,1]);
a[b[i,0],b[i,1]]:=true;
end;
for i:=1 to n do free[i]:=true;
close(input);
end;
function pop:longint;
begin
pop:=st[last];
dec(last);
end;
procedure push(i:longint);
begin
inc(last);
st[last]:=i;
end;
function min(u,v:longint):longint;
begin
if u<v then min:=u else min:=v;
end;
procedure visit(x:longint);
var i:longint;
begin
inc(dem); num[x]:=dem; low[x]:=dem;
push(x);
for i:=1 to n do
if free[i] and a[x,i] then
if num[i]>0 then low[x]:=min(low[x],num[i])
else
begin
visit(i);
low[x]:=min(low[x],low[i]);
end;
if num[x]=low[x] then
begin
inc(sm); d[x]:=sm;
repeat
i:=pop;
free[i]:=false;
d[i]:=sm;
until i=x;
end;
end;
procedure pr;
var i,j,t,u,v:longint;
begin
for i:=1 to n do
if free[i] then visit(i);
if sm=1 then
begin
for i:=1 to n do
for j:=1 to n do
if (i<>j) and not a[i,j] then
begin
ri:=i; rj:=j;
end;
exit;
end;
for i:=1 to m do
begin
if d[b[i,0]]=d[b[i,1]] then continue;
if not e[d[b[i,0]]] then
begin
e[d[b[i,0]]]:=true;
inc(ne);
end;
if not f[d[b[i,1]]] then
begin
f[d[b[i,1]]]:=true;
inc(nf);
end;
end;
if (ne=sm-1) and (nf=sm-1) then
begin
for u:=1 to n do
if not e[u] then break;
for ri:=1 to n do
if d[ri]=u then break;
for v:=1 to n do
if not f[v] then break;
for rj:=1 to n do
if d[rj]=v then break;
end
else re:=1;
end;
procedure wf;
begin
assign(output,fo); rewrite(output);
if re=1 then writeln('NO')
else
begin
writeln('YES');
writeln(ri,' ',rj);
end;
close(output);
end;
begin
rf;
pr;
wf;
end.