1137 N-th Tribonacci Number

🔀 Pick One

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 <= 231 - 1.
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class Solution:
    def __init__(self):
        n = 38
        self.nums = [0] * n
        self.nums[1] = self.nums[2] = 1
        def trib(k):
            if k == 0:
                return 0
            if self.nums[k]:
                return self.nums[k]
            self.nums[k] = trib(k-1) + trib(k-2) + trib(k-3)
            return self.nums[k]
        trib(n-1)
            
    
    def tribonacci(self, n: int) -> int:
        return self.nums[n]