WA成了10分？？

#include<bits/stdc++.h>
using namespace std;
int n,ans=0,lit[20],ed; 
void dfs(int k,int a,int b,int c){
	if(a==ed){++ans;return;}
	for(register int i=ed&(~(a|b|c|lit[k]));i;i-=(i&-i)){
		int x=i&-i;
		dfs(k+1,a+x,(b+x)>>1,(c+x)<<1);
	}	
}
int main(){
    std::ios::sync_with_stdio(false);
    cin>>n;ed=(1<<n)-1;
    string tmp;memset(lit,0,sizeof(lit));
    for(register int i=1;i<=n;++i){
    	cin>>tmp;tmp=' '+tmp;
    	for(register int j=1;j<=n;++j)
    		tmp[j]=='.'?lit[i]|=1<<(n-j):0; 
	}
    dfs(1,0,0,0);
    cout<<ans<<endl;
	return 0;
}