input: 
  Write a function to implement the Fibonacci sequence. The function should return the nth Fibonacci number. If n is 0, return 0; if n is 1, return 1; otherwise, return the sum of the previous two Fibonacci numbers.
  
  
  def fibonacci_seq(n): 
      if n==0: 
          return 0
      elif n==1: 
          return 1
      else: 
          return fibonacci_seq(n-1)+fibonacci_seq(n-2)
  
  assert fibonacci_seq(0)==0
  assert fibonacci_seq(1)==1
  assert fibonacci_seq(5)==5
  assert fibonacci_seq(10)==55
  assert fibonacci_seq(15)==610
  
output:     
      if n==0: 
          return 0
      elif n==1: 
          return 1
      else: 
          a, b = 0, 1
          for _ in range(2, n+1):
              a, b = b, a + b
          return b
  