LeetCode 342. Power of Four

原题链接在这里：https://leetcode.com/problems/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?

题解：

与Power of Two, Power of Three类似。

每次iteration若是不能被4整除即return false. 若可以被4整除，便除以4, 直到结果等于1.

Time Complexity: O(log(num)). Space: O(1).

AC Java:

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

Follow up 需要no loops/recursion.

可以bit manipulation, 首先判定num是否为2的幂数. 若是2的幂数, num二进制表达首位为1, num-1除首位均为1. num & num-1 应等于 0.

然后判定首位的1是在奇数位置上, eg. 10000 = 16. 所以 num & 0x55555555 应等于num 原值.

Time Complexity: O(1). Space: O(1).

public class Solution {
    public boolean isPowerOfFour(int num) {
        return num>0 && (num & num-1) == 0 && (num & 0x55555555) == num;
    }
}