Reverse bits of a given 32 bits unsigned integer.

Example:

Input:
 43261596

Output:
 964176192

Explanation: 
43261596 represented in binary as 
00000010100101000001111010011100
, 
             return 964176192 represented in binary as 
00111001011110000010100101000000
.

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

public class Solution {
    // you need treat n as an unsigned value
    public int reverseBits(int n) {

        int ans = 0;
        for (int i = 0; i < 32; i++){
            ans = (ans << 1) + (n & 1);
            n >>= 1;
        }

        return ans;
    }
}