input: Write a python function to optimize runtime using memoization for computing Fibonacci numbers.
  assert fibonacci(10) == 55
  assert fibonacci(20) == 6765
  assert fibonacci(30) == 832040
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]