Determine if a given integer is power of 2.

Examples

• 16 is power of 2 (2 ^ 4)
• 3 is not
• 0 is not
• -1 is not

Solution: 1. 是2的power的只有1位是1，其余为0； 2.用num & (num - 1)把最右端的1变为0，若是2的power，结果为0