题目：获取斐波那契数列,实际上是triple吹波那契数列:)

The Tribonacci sequence Tn is defined as follows: T0 = 0, T1 = 1, T2 = 1, and Tn+3 = Tn + Tn+1 + Tn+2 for n >= 0. Given n, return the value of Tn.

Example 1:
Input: n = 4
Output: 4
Explanation:
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4
Example 2:
Input: n = 25
Output: 1389537

Constraints: 0 <= n <= 37 The answer is guaranteed to fit within a 32-bit integer, ie. answer <= 2^31 - 1.

我的答案

    public static int tribonacci(int n) {
        switch (n) {
            case 0:
                return 0;
            case 1:
            case 2:
                return 1;
            default:
                return tribonacci(n - 1) + tribonacci(n - 2) + tribonacci(n - 3);
        }
    }
    // 下面这个效率更高
    public int tribonacci(int n) {
        if (n < 2) return n;
        int a = 0, b = 1, c = 1, d;
        while (n-- > 2) {
            d = a + b + c;
            a = b;
            b = c;
            c = d;
        }
        return c;
    }

我的分析

状态转移方程 对于每个数，它都是前面3个数的和
临界退出条件 最初始写死的3个值