509-Fibonacci-Number.cpp

#define IOS ios_base::sync_with_stdio(false), cin.tie(NULL), cout.tie(0);
class Solution {
public:
    int fib(int n) {
        IOS
        if (n <= 1)
            return n;

        int n1 = 0, n2 = 1, n3;
        for (int i = 2; i <= n; i++) {
            n3 = n1 + n2;
            n1 = n2;
            n2 = n3;
        }
        return n3;
    }
};

/*
## Recursion Solution

int fib(int n) {
    if (n <= 1)
        return n;

    return fib(n - 1) + fib(n - 2);
}
*/