AC:

#include <iostream>
using namespace std;
typedef unsigned long long ull;
ull n, k;
ull mypow(int x) {
	ull ret = 1;
	for (int i = 1; i <= x; i++) {
		ret *= 2;
	}
	return ret;
}
int main() {
	cin >> n >> k;
	for (int i = n; i >= 1; i--) {
		ull num2 = mypow(i) - 1;
		ull num = num2 / 2;
		if (k <= num) {
			cout << 0;
		} else {
			cout << 1;
			k = num2 - k;
		}
	}
	return 0;
}

把 mypow(i) 改成 (1 << i) 就挂了，请问这俩有啥区别吗？