509.fibonacci-number.jl

# ---
# title: 509. Fibonacci Number
# id: problem509
# author: Indigo
# date: 2021-01-17
# difficulty: Easy
# categories: Array
# link: <https://leetcode.com/problems/fibonacci-number/description/>
# hidden: true
# ---
# 
# 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.
# 
# 
## @lc code=start
using LeetCode

function fib(n::Int)::Int
    n <= 1 && return n
    pre, cur = 0, 1
    for _ in 2:n
        pre, cur = cur, pre + cur
    end
    return cur
end

## @lc code=end