这段代码在csp会CE吗？

string是该用双引号还是单引号啊？

#include<cstdio>
#include<string>
#include<cstring>
#include<algorithm>
#define ri register int
typedef unsigned long long ull;
using namespace std;
ull n,k;
string dfs(ull n,ull k)
{
	if(n==1) return !k?"0":"1";
	if(k<=(1ULL<<(n-1))-1) return "0"+dfs(n-1,k);
	else return "1"+dfs(n-1,(1ULL<<(n-1))-1+(1ULL<<(n-1))-k);
}
int main()
{
	scanf("%llu %llu",&n,&k);
	printf("%s",dfs(n,k).c_str());	
	return 0;	
}