long sumXor(long n) {
    if (n == 0) {
        return 1;
    }
    
    int num_bits = std::floor(std::log2(n));
    int num_one_bits = __builtin_popcountl(n) - 1; // -1 excludes highest power of 2
    int num_zero_bits = num_bits - num_one_bits;
    
    // # of ways of choosing num_zero_bits
    return 1L << num_zero_bits; // pow(2, num_zero_bits)
}

def sumXor(n):
    count = 1
    while n != 0:
        if n % 2 == 0:
            count *= 2
        n >>= 1
    return count

def sumXor(n):
    return 1 << list(bin(n or 1)[2:][::-1]).count('0')

public static long sumXor(long n)
{
    long mask = n | n >> 1;
    mask |= mask >> 2;
    mask |= mask >> 4;
    mask |= mask >> 8;
    mask |= mask >> 16;
    mask |= mask >> 32;
    return 1L << (int)long.PopCount(mask ^ n);
}