Fibonacci Number

Get the Kth number in the Fibonacci Sequence. (K is 0-indexed, the 0th Fibonacci number is 0 and the 1st Fibonacci number is 1).

Examples

0th fibonacci number is 0
1st fibonacci number is 1
2nd fibonacci number is 1
3rd fibonacci number is 2
6th fibonacci number is 8

Corner Cases

What if K < 0? in this case, we should always return 0.
Is it possible the result fibonacci number is overflowed? We can assume it will not be overflowed when we solve this problem on this online judge, but we should also know that it is possible to get an overflowed number, and sometimes we will need to use something like BigInteger.

首先想到的是用recursion来做，但是发现会出现recursion次数过多的的情况，之后改用数组存储的方式完成更加方便

Recursion：

class Solution(object):
  def fibonacci(self, K):
    if K <= 0:
      return 0
    if K == 1:
      return 1
    res = self.fibonacci(K-1) + self.fibonacci(K-2)
    return res

array存储：

class Solution(object):
  def fibonacci(self, K):
    num = [0,1]
    if K <= 0:
      return 0
    if K == 1:
      return 1
    for i in range(0,K-1):
      num.append(num[-1] + num[-2])
    return num[-1]