n-th-tribonacci-number_1137.py

# 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.

# ---------------------------------------Runtime 29 ms Beats 87.14% Memory 16.53 MB Beats 31.32%---------------------------------------


class Solution:
    cache = {1: 1}

    def tribonacci(self, n: int) -> int:
        if n <= 0:
            return 0
        if n not in self.cache:
            self.cache[n] = (
                self.tribonacci(n - 1)
                + self.tribonacci(n - 2)
                + self.tribonacci(n - 3)
            )
        return self.cache[n]