## 509. Fibonacci Number

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`

## Rust Solution

``````struct Solution;

impl Solution {
fn fib(n: i32) -> i32 {
let mut a: Vec<i32> = vec![0; 31];
a = 1;
for i in 2..=30 {
a[i] = a[i - 1] + a[i - 2];
}
a[n as usize]
}
}

#[test]
fn test() {
assert_eq!(Solution::fib(2), 1);
assert_eq!(Solution::fib(3), 2);
assert_eq!(Solution::fib(4), 3);
}
``````

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