2的幂次

 1 package algorithm;
 2 
 3 /**
 4  * Created by moi on 2017/10/18.
 5  */
 6 public class PowerOf2 {
 7 
 8     public static int powOf2(int n){
 9         if(n==0){
10             return 1;
11         }
12         if(n>31){
13             throw new RuntimeException("n>31 is too big.");
14         }
15         int base = 2;
16         while(n>1){
17             base = base << 1;
18             System.out.println(base);
19             n--;
20         }
21         return base;
22     }
23 
24     public static boolean checkPowerOf2(int n) {
25         // write your code here
26         boolean flag = true ;
27 
28         if ( n < 1 ){
29             return false;
30         }
31 
32         while( n>= 2 ){
33             // 1的二进制是 最低位为1，其余各位为0，所以这里 n & 1 即n的最低位和 1的最低位1 进行与，如果是偶数，结果就是0，奇数就是1.
34             if ( (n & 1) == 1 ){
35                 flag = false ;
36                 break;
37             }
38             n = n >> 1 ;
39         }
40         return flag ;
41     }
42 
43     public static void main(String[] args) {
44         PowerOf2.powOf2(3);
45     }
46 }