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

implSolution{pubfnfib(n:i32) -> i32{if n == 0 || n == 1{ n }else{Self::fib(n - 1) + Self::fib(n - 2)}}}

2. Iteration

implSolution{pubfnfib(n:i32) -> i32{if n == 0 || n == 1{return n;}letmut pre1 = 1;letmut pre2 = 0;letmut fib_num = 1;for i in2..=n { fib_num = pre1 + pre2; pre2 = pre1; pre1 = fib_num;} fib_num }}