题解 | 76选数

#include <stdio.h>

int main() {
	unsigned long long n = 0;
	scanf("%llu", &n);
	unsigned long long a = 1;
	a = a << 63;
	int i = 0;
	for (i = 0;i < 64;i++) {
		if ((n<<i) & a) {
			break;
		}
	}
	unsigned long long b = 0;
	b = ((unsigned long long)1 << (64 - i)) - 1;
	printf("%llu", b);
	return 0;
}