Determine if it is a power of two

Problem description

Given an integer, write a function to determine if it is a power of two.

Examples

Example 1:
Input: 1
Output: true 
Explanation: 20 = 1

Example 2:
Input: 16
Output: true
Explanation: 24 = 16

Example 3:
Input: 218
Output: false

Solution

　　这道题在剑指Offer上的“二进制中1的个数”的封装。首先可以确定的是2的幂的二进制中最左端只有1，其余为0。若如此，可将n = （n - 1）& n，这样会将n二进制中最右端的1赋值为0.。若n为2的幂的话，进行与运算后，n 会等于0。否则不等于 0.

Code

 class Solution {
    public boolean isPowerOfTwo(int n) {
        if (n <= 0) {
            return false;
        }
        return ((n - 1) & n) == 0;
    }
}