input: Write a function to implement the Fibonacci sequence. The function should take an integer `n` as input and return the nth number in the Fibonacci sequence. If `n` is 0 or 1, the function should return `n` directly. For any other value of `n`, the function should return the sum of the two preceding numbers in the sequence.
  
  assert fibonacci_seq(0)==0
  assert fibonacci_seq(1)==1
  assert fibonacci_seq(2)==1
  assert fibonacci_seq(3)==2
  assert fibonacci_seq(4)==3
  assert fibonacci_seq(5)==5
output: def fibonacci_seq(n):
      if n == 0 or n == 1:
          return n
      a, b = 0, 1
      for _ in range(2, n + 1):
          a, b = b, a + b
      return b