斐波那契数列

class Solution {
    public int fib(int n) {
        final int MOD = 1000000007;
        if (n == 0) {
            return 0;
        }
        if (n == 1) {
            return 1;
        }
        int first = 0, second = 1, res = 0;
        for (int i = 2; i <= n; i++) {
            res = (first + second) % MOD;
            first = second;
            second = res;
        }
        return res;
    }
}