input: Write a python function to check if a number is a power of two.
  assert isPowerOfTwo(16) == True
  assert isPowerOfTwo(31) == False
  assert isPowerOfTwo(64) == True
  assert isPowerOfTwo(0) == False
output: def isPowerOfTwo(n):
      if n <= 0:
          return False
      return (n & (n - 1)) == 0