[LeetCode] 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?

class Solution {
public:
    uint32_t reverseBits2(uint32_t n) {
        n = ((n & 0x0000ffff)<<16) + ((n & 0xffff0000)>>16);
        n = ((n & 0x00ff00ff)<<8) + ((n & 0xff00ff00)>>8);
        n = ((n & 0x0f0f0f0f)<<4) + ((n & 0xf0f0f0f0)>>4);
        n = ((n & 0x33333333)<<2) + ((n & 0xcccccccc)>>2);
        n = ((n & 0x55555555)<<1) + ((n & 0xaaaaaaaa)>>1);
        return n;
    }

    uint32_t reverseBits1(uint32_t n) {
        int size = 32;
        int res = 0;
        for(int i=0;i<size;i++)
        {
            int val = ((1<<i)&n)>>i;
            res |= (val<<(size-1-i));
        }
        return res;
    }
};