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).
If this function is called many times, how would you optimize it?
Related problem: Reverse Integer
Special thanks to @ts for adding this problem and creating all test cases.