怎样知道一个数字是不是2的乘方？

最新推荐文章于 2025-08-09 12:51:47 发布

原创最新推荐文章于 2025-08-09 12:51:47 发布 · 1.1k 阅读

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本文介绍三种方法来判断一个正整数是否为2的幂次方：使用正则表达式匹配二进制字符串中仅有的一个1；通过位运算检查数字与其减一后的结果；利用模运算递归地除以2直到结果为1或出现奇数。

回答了上一篇日志中的一个问题：怎样知道一个数字是不是2的乘方？

在网上搜索了一下，自己总结后，整理出三个方法，代码如下：

package testPass; /** *for positive integer only! *@author 孙如意 rysun@qq.com */ public class IsPowerOfTwo { public static void main (String args[]){ System.out.println("the method of regular expersion: 128 is a power of 2 ? " + method1(128)); System.out.println("the method of bit operation: 128 is a power of 2 ? " + method2(128)); System.out.println("the method of mod operation: 128 is a power of 2 ? " + method3(128)); } /** * based on regular expression * @param num * @return wheather the num is a power of two */ public static boolean method1(Integer num){ String binNum = Integer.toBinaryString(num); //a number which is a power of 2 in binary has only one 1 and starts with it return binNum.matches("10*"); } /** * based on bit operation * @param num * @return wheather the num is a power of two */ public static boolean method2(Integer num) { if ((num & (num-1)) == 0) { return true; } return false; } /** * based on mod operation * @param num * @return wheather the num is a power of two */ public static boolean method3 (Integer num) { if(num > 0){ while(num > 0){ if (num % 2 == 1) { return false; }else { num /= 2; if(num == 1 ) { return true; } } } } return false; } }