342. Power of Four

题目

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?

链接

https://leetcode.com/problems/power-of-four/?tab=Description

3/8/2017

没有想到follow up的方法

public class Solution {
    public boolean isPowerOfFour(int num) {
        if (num <= 0) return false;
        while (num != 1) {
            if (num % 4 != 0) return false;
            num /= 4;
        }
        return true;
    }
}

他人的好算法

    public boolean isPowerOfFour(int num) {
        return num > 0 && (num&(num-1)) == 0 && (num & 0x55555555) != 0;
        //0x55555555 is to get rid of those power of 2 but not power of 4
        //so that the single 1 bit always appears at the odd position 
    }

另外一个，可以用于任何power的例子，只需要把底数改了即可。

public boolean isPowerOfFour(int num) {
    return Integer.toString(num, 4).matches("10*");
}