342. Power of Four

 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?

public class Solution { public boolean isPowerOfFour(int num) { // double res=Math.log(num)/Math.log(4); // return Math.abs((res-(int)res))<0.0000000001; if(num<0) return false; return (num&(num-1))==0&&(num&0x55555555)==num;//1个数是4的n次方一定是2的n次方，但是反过来不一定成立//所以判断4的n次方还得再加一个条件，4的幂只能是奇数位为1，而2的幂只有有一个位置为1就行 //还有一个小tips，如果nums是2的n次方，那么nums&(nums-1)=0 } }