Reverse Bits

Reverse bits of a given 32 bits unsigned integer.

For example, given input 43261596 (represented in binary as 00000010100101000001111010011100), return 964176192 (represented in binary as00111001011110000010100101000000).

Follow up:
If this function is called many times, how would you optimize it?

Related problem: Reverse Integer


My code:
 1 public class ReverseBits {
 2 
 3     public static void main(String[] args) {
 4         // TODO Auto-generated method stub
 5         ReverseBits test = new ReverseBits();
 6         
 7         System.out.println(test.reverseBits(43261597));
 8     }
 9 
10     // Accepted
11       public long reverseBits(int n) {
12           
13           String str = Integer.toBinaryString(n);
14           StringBuilder sb = new StringBuilder();
15           int len = str.length();
16          
17               for(int i = len -1; i >=0 ; i--)
18               {
19                   sb.append(str.charAt(i));
20               }
21              for(int i = len; i < 32; i++)
22               {
23                   sb.append("0");
24               }
25              
26              long  base = 1;
27              long re = 0;
28           for(int i = 31 ; i >=0; i--)
29           {
30               if( sb.charAt(i) == '1')
31               {
32                   re += base;
33               }
34               base *=2;
35           }
36           return re;
37           }
38 }