342. Power of Four【位运算】

2017/3/23 22:23:57

Given an integer (signed 32 bits), write a function to check whether it is a power of 4.

Example:
Given num = 16, return true. Given num = 5, return false.

Follow up: Could you solve it without loops/recursion?

1、指数运算是必须大于0；

2、首先满足2的指数（只有一个bit是1），利用num&num-1==0即可判断；

3、去掉只满足2的指数不满足4的指数的值，0x2AAAAAAA对应的bit位不能是1。100B = 4 属于4的指数，1000B=8仅仅是2的指数。

版本1  Java   O(1)

1. publicclassSolution{
2. publicboolean isPowerOfFour(int num){
3. return num >0&&( num & num -1 |num &0x2AAAAAAA )==0;
4. }
5. }