题目：
平面上给定N个两两不同的整点，统计以给定的点为顶点，且直角边平行于坐标轴的直角三角形数。
分析：
直接枚举每个点坐标即可。
附上代码：
type arr=array [1..100002] of longint; var x,y,x1,y1,x2,y2,numx,numy:arr; n:longint; procedure qsort(l,r:longint; var a:arr); var i,j,mid,temp:longint; begin if l>=r then exit; mid:=a[l+random(r-l+1)];i:=l;j:=r; repeat while a[i]<mid do inc(i); while a[j]>mid do dec(j); if i<=j then begin temp:=a[i];a[i]:=a[j];a[j]:=temp; inc(i);dec(j); end; until i>j; qsort(l,j,a); qsort(i,r,a); end; procedure init; var i:longint; begin readln(n); for i:=1 to n do begin readln(x[i],y[i]); x1[i]:=x[i]; y1[i]:=y[i]; end; end; procedure main; var i,k,k1,temp,l1,r1,t,mid,ans,num1,num2:longint; begin ans:=0;qsort(1,n,x);qsort(1,n,y);k:=1;x2[k]:=x[1]; for i:=2 to n do begin if x[i]=x[i-1] then inc(numx[k]) else begin inc(k); x2[k]:=x[i]; end; end; k1:=1; y2[k1]:=y[1]; for i:=2 to n do begin if y[i]=y[i-1] then inc(numy[k1]) else begin inc(k1); y2[k1]:=y[i]; end; end; x:=x1; y:=y1; for i:=1 to n do begin temp:=x[i];t:=y[i];l1:=1;r1:=k; while l1<=r1 do begin mid:=(l1+r1) shr 1; if x2[mid]=temp then break; if x2[mid]>temp then r1:=mid-1 else l1:=mid+1; end; if x2[mid]=temp then num1:=numx[mid]; l1:=1;r1:=k1; while l1<=r1 do begin mid:=(l1+r1) shr 1; if y2[mid]=t then break; if y2[mid]>t then r1:=mid-1 else l1:=mid+1; end; if y2[mid]=t then num2:=numy[mid]; ans:=ans+num1*num2; end; writeln(ans); end; begin init; main; end.
