Leetcode 1137. N-th Tribonacci Number

Problem

The Tribonacci sequence $T_n$ is defined as follows:

$T_0=0,T_1=1,T_2=1$, and $T_{n+3}=T_n+T_{n+1}+T_{n+2}$ for $n>=0.$

Given $n$, return the value of $T_n$.

Algorithm

Calculated using the recursive formula.

Code

class Solution:
    def tribonacci(self, n: int) -> int:
        if n < 2:
            return n
        if n == 2:
            return 1 

        Tn_3, Tn_2, Tn_1, = 0, 1, 1
        for i in range(3, n+1):
            Tn = Tn_1 + Tn_2 + Tn_3
            Tn_3 = Tn_2
            Tn_2 = Tn_1
            Tn_1 = Tn
        
        return Tn