public class Solution {

// you need treat n as an unsigned value

public int reverseBits(int n) {

int result =0;

for(int i=0;i<32;i++)

{

result+=n&1;

n>>=1;

if(i<31)

result<<=1;

}

return result;

}

}

-------------------------------------

// this is a optimization

// cache

private final Map<Byte, Integer> cache = new HashMap<Byte, Integer>();

public int reverseBits(int n) {

byte[] bytes = new byte[4];

for (int i = 0; i < 4; i++) // convert int into 4 bytes

bytes[i] = (byte)((n >>> 8*i) & 0xFF);

int result = 0;

for (int i = 0; i < 4; i++) {

result += reverseByte(bytes[i]); // reverse per byte

if (i < 3)

result <<= 8;

}

return result;

}

private int reverseByte(byte b) {

Integer value = cache.get(b); // first look up from cache

if (value != null)

return value;

value = 0;

// reverse by bit

for (int i = 0; i < 8; i++) {

value += ((b >>> i) & 1);

if (i < 7)

value <<= 1;

}

cache.put(b, value);

return value;

}