1137. 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.

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class Solution:
    def tribonacci(self, n: int) -> int:
        
        alist={}
        alist[0]=0
        alist[1]=1
        alist[2]=1
        for i in range(3,n+1):
            alist[i]=alist[i-3]+alist[i-2]+alist[i-1]
        return alist[n]