190. Reverse Bits

Reverse bits of a given 32 bits unsigned integer.

Example:

Input: 43261596
Output: 964176192
Explanation: 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:
    # @param n, an integer
    # @return an integer
    def reverseBits(self, n):
        a = bin(n)
        a = a.split("b")[1]
        a = "0"*(32-len(a)) + a
        a = a[::-1]
        a = int(a,2)
        return a