LeetCodeEasy- [Interview Question 10- I. Fibonacci Sequence]

Write a function, enter n and find the nth term of the Fibonacci sequence. The definition of the Fibonacci sequence is as follows:

F (0) = 0, F (1) = 1
F (N) = F (N-1) + F (N-2), where N> 1. The
Fibonacci sequence starts from 0 and 1, and then The Fibonacci number is obtained by adding two previous numbers.

The answer needs to be modulo 1e9 + 7 (1000000007). If the initial result of the calculation is: 1000000008, please return 1.

Example 1:

Input: n = 2
Output: 1

Example 2:

Input: n = 5
Output: 5

prompt:

0 <= n <= 100

Source: LeetCode
Link: https://leetcode-cn.com/problems/fei-bo-na-qi-shu-lie-lcof The
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Idea 1: Recursion

Just follow the definition and calculate it later.

class Solution:
    def fib(self, n: int) -> int:
        if n == 0:
            return 0
        elif n == 1:
            return 1
        a = 0
        b = 1
        for i in range(2, n+1):
            a, b = b, a + b # python交换两个数
        return b % 1000000007

Idea 2: Recursion

Adding memory array still times out

class Solution:
    def fib(self, n: int) -> int:
        self.data = [0] * (n+1)
        self.data[1] = 1
        return self.f(n)
    def f(self, n):
        if n == 0:
            return 0
        elif n == 1:
            return 1
        if self.data[n] != 0:
            return self.data[n]
        return (self.f(n-1) + self.f(n-2)) % 1000000007