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] = 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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