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?

public class Solution {
    // you need treat n as an unsigned value
    public int reverseBits(int n) {
        for(int i = 0; i < 16; i ++){
            int j = 31 - i;
            if(((n >> i) & 1) != ((n >> j) & 1)){
                n ^= (1 << i) | (1 << j);
            }
        }
        return n;
    }
}

Swap two bit in the Integer:

public int swapBits(int val, int i, int j){
    if(((val >> i) & 1) != ((val >> j) & 1)){
        val ^= (1 << i) | (1 << j);
    }
    return val;
}