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 (Java)

1. Brute Force

class Solution {
    public int fib(int N) {
        int pre = 0;
        int cur = 1;
        int tmp = 0;
        
        if(N == 0)
            return pre;
        else if(N == 1)
            return cur;

        for(int i = 2; i <= N; i++){
            tmp = cur;
            cur = pre + tmp;
            pre = tmp;
        }

        return cur;
    }
}

2. Arrays

class Solution {
    public int fib(int N) {
        int[] F = {0, 1, 1};
        
        if(N < 3)
            return F[N];

        for(int i = 3; i <= N; i++){
            F[0] = F[1];
            F[1] = F[2];
            F[2] = F[0] + F[1];
        }

        return F[2];
    }
}