input: Write a python function to generate all binary strings of length n.
  assert generate_binary_str(2) == ['00', '01', '10', '11']
  assert generate_binary_str(3) == ['000', '001', '010', '011', '100', '101', '110', '111']
  assert generate_binary_str(1) == ['0', '1']
output: def generate_binary_str(n):
      if n == 0:
          return ['']
      else:
          smaller = generate_binary_str(n - 1)
          return ['0' + bitstring for bitstring in smaller] + ['1' + bitstring for bitstring in smaller]