input: Write a python function to simulate a series of coin tosses and count the number of heads and tails.
  assert simulate_tosses(10) in ['HHHHHHHHHH', 'HHHHHHHHHT', 'HHHHHHHHTH', ..., 'TTTTTTTTTT']
  assert simulate_tosses(0) == ''
  assert simulate_tosses(1) in ['H', 'T']
output: def simulate_tosses(n):
      import random
      result = ''.join(random.choice(['H', 'T']) for _ in range(n))
      return result