342. Power of Four

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

Example 1:

Input: 16
Output: true

Example 2:

Input: 5
Output: false

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

 

M1: loop

time = O(log_4(n)), space = O(1)

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

M2: 换底公式

time = unknown, space = O(1)

class Solution {
    public boolean isPowerOfFour(int num) {
        return (Math.log10(num) / Math.log10(4)) % 1 == 0;
    }
}

M3: bit manipulation

n > 0 and (num & (num - 1)) == 0 makes sure num is a power of 2

0x55555555 == 1010101010101010101010101010101 in binary with a length of 32,  (num & 0x55555555) != 0 makes sure the 1 locates in the odd location

time = O(1), space = O(1)

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