QBPOINT - Bộ ba điểm thẳng hàng
Tác giả: RR
Ngôn ngữ: Pascal
//Wishing myself a happy lunar new year with a lot of accept solutions
{$R+,Q+}
const
FINP='';
FOUT='';
eps=1e-10;
oo=1000000;
MAXN=2000;
var
x,y:array[1..MAXN] of longint;
tan:array[1..MAXN] of double;
n,sl,kq:longint;
procedure inp; inline;
var
f:text;
i:longint;
begin
assign(f,FINP); reset(f);
read(f,n);
for i:=1 to n do read(f,x[i],y[i]);
close(f);
end;
procedure swap(var a,b:double); inline;
var
temp:double;
begin
temp:=a; a:=b; b:=temp;
end;
procedure sort(l,r:longint); inline;
var
i,j:longint;
mid,tg:double;
begin
i:=l; j:=r; mid:=tan[(l+r) div 2];
repeat
while (tan[i]<mid) do inc(i);
while (tan[j]>mid) do dec(j);
if i<=j then
begin
swap(tan[i],tan[j]);
inc(i); dec(j);
end;
until i>j;
if l<j then sort(l,j);
if i<r then sort(i,r);
end;
procedure find(u:longint); inline;
var
v,i,count:longint;
begin
sl:=0;
for v:=u+1 to n do
begin
inc(sl);
if (x[u]=x[v]) then tan[sl]:=oo
else if (y[u]=y[v]) then tan[sl]:=0
else tan[sl]:=(y[v]-y[u])/(x[v]-x[u]);
end;
if sl=0 then exit;
sort(1,sl);
count:=1;
for i:=2 to sl do
begin
if abs(tan[i]-tan[i-1])<eps then inc(count)
else
begin
kq:=kq+(count*(count-1)) shr 1;
count:=1;
end;
end;
kq:=kq+(count*(count-1)) shr 1;
end;
procedure solve; inline;
var
i,j:longint;
begin
kq:=0;
for i:=1 to n do find(i);
end;
procedure ans; inline;
var
f:text;
begin
assign(f,FOUT); rewrite(f);
writeln(f,kq);
close(f);
end;
begin
inp;
solve;
ans;
end.