1137. N-th Tribonacci Number

Problem Link

Description

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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 = 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.

Solution

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Python3

class Solution:
    def tribonacci(self, n: int) -> int:
        if n == 0: return 0
        if n <= 2: return 1
 
        a, b, c = 0, 1, 1
        for _ in range(n - 3):
            a, b, c = b, c, a + b + c
        
        return a + b + c

C++

class Solution {
public:
    int arr[38];
    int recur(int n){
        if (arr[n] != -1) return arr[n];
        int result = recur(n-1) + recur(n-2) + recur(n-3);
        arr[n] = result;
        return result;
    }
    int tribonacci(int n) {
        fill_n(arr, 38, -1);
        arr[0] = 0;
        arr[1] = 1;
        arr[2] = 1;
        return recur(n);
    }
};

Python

class Solution(object):
    def tribonacci(self, n):
        a, b, c = 1, 0, 0
        for _ in xrange(n): a, b, c = b, c, a + b + c
        return c