leetcode 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.

The famous Fibonacci Numbers ,F(0)=0, F(1)=1,
Followed by the sum of the first two numbers , Please n individual Fibonacci Numbers .

Ideas ：
DP
It can be recorded with a one-dimensional array 0～n individual Fibonacci Numbers , Then the first i One number is F(i-2)+F(i-1),
But because only the first two are used , So just save it with two numbers .

public int fib(int n) {
    
    if(n == 0) return 0;
    if(n == 1) return 1;
    
    int n1 = 0;
    int n2 = 1;
    for(int i = 2; i <= n; i ++) {
    
        int tmp = n1 + n2;
        n1 = n2;
        n2 = tmp;
    }
    
    return n2;
}