input: Write a python function to calculate the Fibonacci number using memoization.
  assert fibonacci(0) == 0
  assert fibonacci(1) == 1
  assert fibonacci(5) == 5
  assert fibonacci(10) == 55
  assert fibonacci(15) == 610
output: def fibonacci(n, memo={}):
      if n in memo:
          return memo[n]
      if n <= 1:
          return n
      memo[n] = fibonacci(n-1, memo) + fibonacci(n-2, memo)
      return memo[n]