Solution

class Solution {
public:
    int Fibonacci(int n) {
        if (n < 2) return n;
        int dp[2] = {0, 1};
        for (int i = 2; i <= n; ++i) {
            int num = dp[0] + dp[1];
            dp[0] = dp[1];
            dp[1] = num;
        }
        return dp[1];
    }
};