Leetcode 190 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?

Solution 1: Bit manipulation. O(1) 

 public class Solution {  
   // you need treat n as an unsigned value  
   public int reverseBits(int n) {  
     int res=0;  
     for (int i=0; i<32; i++) {  
       res=res<<1|(n&1);  
       n>>=1;  
     }  
     return res;  
   }  
 }