# [1137. N-th Tribonacci Number](https://leetcode.com/problems/n-th-tribonacci-number/)


## 题目

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`.


## 题目大意


泰波那契序列 Tn 定义如下： 

T0 = 0, T1 = 1, T2 = 1, 且在 n >= 0 的条件下 Tn+3 = Tn + Tn+1 + Tn+2

给你整数 n，请返回第 n 个泰波那契数 Tn 的值。

提示：

- 0 <= n <= 37
- 答案保证是一个 32 位整数，即 answer <= 2^31 - 1。



## 解题思路

- 求泰波那契数列中的第 n 个数。
- 简单题，按照题意定义计算即可。