LeetCode 509. Fibonacci Number

class Solution {
public:
    int fib(int n) {
        if (n < 2) {
            return n;
        }
        int p = 0, q = 0, r = 1;
        for (int i = 2; i <= n; ++i) {
            p = q; 
            q = r; 
            r = p + q;
        }
        return r;
    }
};