509_Fibonacci_Number.java

class Solution {
    public int fib(int n) {
        if(n<=1){return n;}
        int first = 0;
        int second = 1;
        for(int i = 2; i <= n; i++) {
            int temp = first + second;
            first = second;
            second = temp;
        }
        return second;
    }
}