USACO 6.5.5 Checker Challenge 位运算+dfs

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这题就是n皇后问题，用位运算的dfs秒掉就好了。

{IDL: ymwbegi1PROG: checkerLANG: PASCAL} var  n,ans,s:longint;  f:array[1..13] of longint;procedure print;var  i,s,x:longint;begin  for i:=1 to n do  begin    x:=f[i];    s:=0;    while x>0 do    begin      inc(s);      x:=x div 2;    end;    if i<n      then write(s,' ')      else writeln(s);  end;end;procedure dfs(row,dl,dr,x:longint);var  p,q:longint;begin  if row=1 shl n-1 then  begin    if s<3 then    begin      inc(s);      print;    end;    inc(ans);    exit;  end;  p:=((row or dl or dr) xor (1 shl n-1)) and (1 shl n-1);  while p>0 do  begin    q:=p and (-p);    f[x]:=q;    dfs(row+q,(dl+q) shl 1,(dr+q) shr 1,x+1);    p:=p-q;  end;end;begin  assign(input,'checker.in');  assign(output,'checker.out');  reset(input);  rewrite(output);  readln(n);  dfs(0,0,0,1);  writeln(ans);  close(input);  close(output);end.

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