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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.
1 class Solution {
2 public:
3 bool isPowerOfFour(int num) {
4 return (num > 0) && ((num & (num - 1)) == 0) && ((num & 0x55555555) == num);
5 }
6 };
(num & (num - 1)) == 0 判断这个数是不是2的倍数，(num & 0x55555555) == num 判断是不是4的倍数。0x55555555就是二进制数上奇数位上为1.
Power of Four（Difficulty: Easy）
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原文地址：http://www.cnblogs.com/lrh-xl/p/5462456.html
