#142 O(1) Check Power of 2

题目描述：

Using O(1) time to check whether an integer n is a power of 2.

Have you met this question in a real interview? 

Yes

Example

For n=4, return true;

For n=5, return false;

Challenge 

O(1) time

题目思路：

每个int32都一共只有32位，而如果是2的power，那么这32位中必须只有一位是‘1’。

Mycode（AC = 39ms）：

class Solution {public:    /*     * @param n: An integer     * @return: True or false     */    bool checkPowerOf2(int n) {        // write your code here        long long ln = (long long)n;        if (ln < 0) return false;                // traverse all the 32 bits, the result        // should only contain one '1'        int count = 0;        for (int i = 0; i < 32; i++) {            count += ln % 2;            ln >>= 1;        }                return count == 1;    }};