LeetCode 509 Fibonacci Number - Easy

509. Fibonacci Number — Easy

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Problem

The Fibonacci numbers, commonly denoted F(n) form a sequence, called the Fibonacci sequence, such that each number is the sum of the two preceding ones, starting from 0 and 1. That is,

F(0) = 0, F(1) = 1 F(N) = F(N - 1) + F(N - 2), for N > 1. Given N, calculate F(N).

Example 1:

Input: 2 Output: 1 Explanation: F(2) = F(1) + F(0) = 1 + 0 = 1. Example 2:

Input: 3 Output: 2 Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2. Example 3:

Input: 4 Output: 3 Explanation: F(4) = F(3) + F(2) = 2 + 1 = 3.

Note:

0 ≤ N ≤ 30.

Solution

### Recursion O(2^n)
class Solution:
    def fib(self, n):
        if n <= 1:
            return n
        return self.fib(n - 1) + self.fib(n - 2)
        
 
### DP O(n)
class Solution:
    def fib(self, n):
        if n <= 1:
            return n
        
        f = [0] * (n + 1)
        f[0] = 0
        f[1] = 1
        for i in range(2, n + 1):
            f[i] = f[i - 1] + f[i - 2]
        
        return f[n]