N-th Tribonacci Number

N-th Tribonacci Number

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.

Solutions

🧠 Cpp

class Solution
{
    //max n == 38
     array<int, 38> _storage = {0,1,1};
public:
    constexpr int tribonacci(int n)
    {
        if(n && !_storage[n])
            _storage[n] = 
            tribonacci(n-1) + tribonacci(n-2) + tribonacci(n-3);

        return _storage[n];
    }
};