leetcode-326-easy

Power of Four

Given an integer n, return true if it is a power of four. Otherwise, return false.

An integer n is a power of four, if there exists an integer x such that n == 4x.

Example 1:

Input: n = 16
Output: true
Example 2:

Input: n = 5
Output: false
Example 3:

Input: n = 1
Output: true
Constraints:

-231 <= n <= 231 - 1
Follow up: Could you solve it without loops/recursion?

思路一：用 326 题的解法

    public boolean isPowerOfFour(int n) {
        while (n > 0 && n % 4 == 0) {
            n /= 4;
        }

        return n == 1;
    }

思路二：看了一下官方解法，是用位运算解决的

    public boolean isPowerOfFour(int n) {
        return n > 0 && (n & (n - 1)) == 0 && (n & 0xaaaaaaaa) == 0;
    }

    public boolean isPowerOfFour(int n) {
        return n > 0 && (n & (n - 1)) == 0 && n % 3 == 1;
    }