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# 0509. Fibonacci Number

<https://leetcode.com/problems/fibonacci-number>

## Description

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`

## ac

```java
```