LeetCode 1137. N-th Tribonacci Number

Description

https://leetcode.com/problems/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.

Explanation

Implement base on the tribonacci definition. Use a list to reduce repeating computations.

Python Solution

class Solution:
    def tribonacci(self, n: int) -> int:
        if n == 0:
            return 0
        
        if n == 1:
            return 1
        
        if n == 2:
            return 1
        

        f = [0, 1, 1]
        
        for i in range(3, n + 1):
            f.append(f[i - 2] + f[i - 1] + f[i - 3])
            
        
        return f[n]
  • Time Complexity: O(N).
  • Space Complexity: O(N).

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