class Solution {
public:
    int fib(int n) {
        if (n == 0) return 0;
        if (n == 1) return 1;
        if (n == 2) return 1;
        int a = 1, b = 1;
        for (int i = 3; i <= n; i ++) {
            int tmp = a + b;
            a = b;
            b = tmp;
        }
        return b;
    }
};

class Solution {
public:
    int fib(int n) {
        int a = 0, b = 1;
        for (int i = 0; i < n; i ++) {
            int sum = a + b;
            a = b;
            b = sum;
        }
        return a;
    }
};

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