在6*6的方格地盘中，种植24颗树，使每行、每列都有4颗树。求出所有可能的种植方案

var
a : array[1..6,1..6] of byte;
t : longint;
h,z : array[1..6] of byte;

procedure sub(s,y,e : byte);
var
x,i,j : byte;
begin
if s > 24
then begin
inc(t);
writeln('No . ',t);
for i := 1 to 6 do begin
for j := 1 to 6 do write(a[i,j]:4);
writeln;
end;
readln;
end
else for x := e to 6 do
if (a[y,x] = 0) and (h[y] < 4) and (z[x] < 4) then begin
a[y,x] := 1; inc(h[y]); inc(z[x]);
if s mod 4 = 0
then sub(s+1,y+1,1)
else sub(s+1,y,x+1);
a[y,x] := 0; dec(h[y]); dec(z[x]);
end;
end;

begin
t := 0;
fillchar(a,sizeof(a),0);
fillchar(h,sizeof(h),0);
z := h;
sub(1,1,1);
writeln('Total = ',t