leetcode-1137 - N-th Tribonacci Number

動機

複習fib

Problem

The Tribonacci sequence T_n is defined as follows: 

T₀ = 0, T₁ = 1, T₂ = 1, and T_n+3 = T_n + T_n+1 + T_n+2 for n >= 0.

Given n, return the value of T_n.

Example 1:

Input: n = 4Output: 4Explanation:T_3 = 0 + 1 + 1 = 2T_4 = 1 + 1 + 2 = 4

Example 2:

Input: n = 25Output: 1389537

Constraints:

• 0 <= n <= 37
• The answer is guaranteed to fit within a 32-bit integer, ie. answer <= 2^31 - 1.

Sol

class Solution:
    @cache
    def tribonacci(self, n: int) -> int:
        if n == 0:
            return 0
        elif n in [1,2]:
            return 1
        else:
            return self.tribonacci(n-3)+self.tribonacci(n-2)+self.tribonacci(n-1)