# 1137. N-th Tribonacci Number

## Description

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
### 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`.

## 思路
像是 fibonacci 數列一樣，使用一個陣列保存，一直取前三項相加，取陣列中 index n 的便是答案。

## C++

class Solution {
public:
    int tribonacci(int n) {
        int tmp[38];
        tmp[0] = 0;
        tmp[1] = 1;
        tmp[2] = 1;
        if (n < 3)
            return tmp[n];
        for (int i = 3; i <= n; i++) {
            tmp[i] = tmp[i - 1] + tmp[i - 2] + tmp[i - 3];
        }
        return tmp[n];
    }
};