input: Write a python function to find the largest Fibonacci number less than a given number.
  assert largestFibonacciNumber(10) == 8
  assert largestFibonacciNumber(50) == 34
  assert largestFibonacciNumber(100) == 89
output: def largestFibonacciNumber(n):
      if n <= 1:
          return 0
      a, b = 0, 1
      while b < n:
          a, b = b, a + b
      return a