2-3

public class Solution {
    // you need treat n as an unsigned value
    public int reverseBits(int n) {
        int result = 0;
        for (int i = 0; i <= 31; i++) {
            int bit = (n >> i) & 1;
            if (bit == 1) { // If n's i-th bit is true, then
                result |= 1 << (31 - i); // set true the result's (31-i)-th bit.
            }
        }
        return result;
    }
}