BZOJ 1050： [HAOI2006]旅行comf（枚举+并查集）

program comf;
var
  f:array[0..500]of longint;
  a,b,c:array[0..5000]of longint;
  n,i,m,l,r,s,t,ans,j,x,y:longint;
function gcd(x,y:longint):longint;
var r:longint;
begin
  r:=x mod y;
  while r<>0 do
   begin x:=y; y:=r; r:=x mod y; end;
  exit(y);
end;
function find(x:longint):longint;
var i,j,k:longint;
begin
  i:=x; j:=x;
  while i<>f[i] do i:=f[i];
  while i<>j do begin k:=f[j]; f[j]:=i; j:=k; end;
  exit(i);
end;
procedure cheak(x,y:longint);
var t:longint;
begin
  if x*r<y*l then begin  t:=gcd(x,y); l:=x div t; r:=y div t; end;
end;
procedure qsort(l,h:longint);
var i,j,t,m:longint;
begin i:=l; j:=h;
 m:=c[(i+j) div 2];
 repeat
while c[i]<m do inc(i);
while m<c[j] do dec(j);
if i<=j then
  begin t:=a[i]; a[i]:=a[j]; a[j]:=t;t:=b[i]; b[i]:=b[j]; b[j]:=t;
    t:=c[i]; c[i]:=c[j]; c[j]:=t;
inc(i); dec(j);  end;  until i>j;
 if i<h then qsort(i,h); if j>l then qsort(l,j);  end;
begin
  readln(n,m);
  for i:=1 to m do
    readln(a[i],b[i],c[i]);
  qsort(1,m); l:=2; r:=0; readln(s,t);
  for i:=1 to m do
   begin
     ans:=-1;
     for j:=1 to n do f[j]:=j;
     for j:=i to m do
      begin
        x:=find(a[j]); y:=find(b[j]);
        f[y]:=x;
        x:=find(s); y:=find(t);
        if x=y then begin ans:=j; break; end;
      end;
     if ans>-1 then cheak(c[ans],c[i]);
   end;
  if r=0 then writeln(‘IMPOSSIBLE‘)
   else if r=1 then writeln(l) else writeln(l,‘/‘,r); 
end.