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 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 res = 0;        for (int i = 0; i < 32; i++) {            res += (n & 1);            n >>>= 1;            if (i < 31) res <<= 1;        }        return res;    }}

class Solution {public:    uint32_t reverseBits(uint32_t n) {        uint32_t res = 0;        for (int i = 0; i < 32; i++)            res = (res << 1) + (n >> i & 1);        return res;    }};