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。
题解
我们发现是找两条线段的交点，分别储存横纵坐标，数据大用HASH。
代码：
varx,y,x1,y1,h,hh:array[-1500000..1500000] of longint;a:array[1..100000,1..2] of longint;i,j,n,s,m,m1,ans:longint;
function h0(a:longint):longint;
varb:longint;
beginb:=a mod 403219;while (h[b]<>0)and(h[b]<>a) dob:=b mod 403219+1;h0:=b;
end;
function h1(a:longint):longint;
varb:longint;
beginb:=a mod 403219;while (hh[b]<>0)and(hh[b]<>a) dob:=b mod 403219+1;h1:=b;
end;
beginreadln(n);for i:=1 to n dobeginreadln(a[i,1],a[i,2]);j:=h0(a[i,2]);if h[j]<>a[i,2] thenbeginm:=m+1;x[j]:=m;h[j]:=a[i,2];y[x[j]]:=y[x[j]]+1;endelsey[x[j]]:=y[x[j]]+1;j:=h1(a[i,1]);if hh[j]<>a[i,1] thenbeginm1:=m1+1;x1[j]:=m1;hh[j]:=a[i,1];y1[x1[j]]:=y1[x1[j]]+1;endelsey1[x1[j]]:=y1[x1[j]]+1;end;for i:=1 to n doans:=ans+(y[x[h0(a[i,2])]]-1)*(y1[x1[h1(a[i,1])]]-1);writeln(ans);
end.