leetcode-Reverse Bits

Difficulty: Easy
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 reverseBits(uint32_t n) {
int      size=sizeof(n)*8;
uint32_t res=0;
for(int i=0;i<size;++i){
if(n&(1<<i))
res|=(1<<(size-i-1));
}
return res;
}
};