注意sum的改变

共2^20种情况，应该也可以用位运算

Program P2531;
Var
   n,i,j:longint;
   a:array[1..20,1..20] of longint;
   b:array[1..20] of boolean;
   ans:longint;
procedure dfs(k,sum:longint);
var
   i,j,sum2:longint;
begin
   b[k]:=true;
   sum2:=sum;
   for i:=1 to n do
      if b[i] then dec(sum2,a[i,k])
      else inc(sum2,a[i,k]);
   if (ans<sum2) then ans:=sum2;
   for i:=k+1 to n do
   begin
      dfs(i,sum2);
   end;

   b[k]:=false;
end;
begin
   ans:=-100000;
   read(n);
   fillchar(b,sizeof(b),false);
   for i:=1 to n do
      for j:=1 to n do
         read(a[i,j]);
   dfs(1,0);
   writeln(ans);
end.