# 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`

```python
class Solution:
    def __init__(self):
        self.dp = {}
        self.dp[0] = 0
        self.dp[1] = 1
        self.dp[2] = 1
    
    def fib(self, n: int) -> int:
        if n in self.dp:
            return self.dp[n]
        return self.fib(n-1) + self.fib(n-2)
```