输入：n = 4
输出：4
解释：
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4

示例 2：
输入：n = 25
输出：1389537

class Solution {
public:
    int tribonacci(int n) {
        if (n == 0) return 0;
        if (n == 1 || n == 2) return 1;
        int t1 = 0, t2 = 1, t3 = 1;
        for (int i = 3; i <= n; ++i) {
            int tmp = t1 + t2 + t3;
            t1 = t2;
            t2 = t3;
            t3 = tmp;
        }
        return t3;
    }
};