README.md

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

Example 2:

Input: 3
Output: 2
Explanation: F(3) = F(2) + F(1) = 1 + 1 = 2.

Example 3:

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

Note:

0 ≤ N ≤ 30.

Solutions (Rust)

1. Recursion

impl Solution {
    pub fn fib(n: i32) -> i32 {
        if n == 0 || n == 1 {
            n
        } else {
            Self::fib(n - 1) + Self::fib(n - 2)
        }
    }
}

2. Iteration

impl Solution {
    pub fn fib(n: i32) -> i32 {
        if n == 0 || n == 1 {
            return n;
        }
        let mut pre1 = 1;
        let mut pre2 = 0;
        let mut fib_num = 1;
        for i in 2..=n {
            fib_num = pre1 + pre2;
            pre2 = pre1;
            pre1 = fib_num;
        }
        fib_num
    }
}