509\. Fibonacci Number

Easy

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:** n = 2

**Output:** 1

**Explanation:** F(2) = F(1) + F(0) = 1 + 0 = 1.

**Example 2:**

**Input:** n = 3

**Output:** 2

**Explanation:** F(3) = F(2) + F(1) = 1 + 1 = 2.

**Example 3:**

**Input:** n = 4

**Output:** 3

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

**Constraints:**

*   `0 <= n <= 30`