1137. N-th Tribonacci Number

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1137. N-th Tribonacci Number

Description

Difficulty: Easy

Related Topics: Math, Dynamic Programming, Memoization

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:

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

Example 2:

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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.

Hints/Notes

  • just do it intuitively

Solution

Language: C++

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class Solution {
public:
    int tribonacci(int n) {
        if (n == 0) return 0;
        if (n == 1 || n == 2) return 1;
        vector<int> res = {0, 1, 1};
        int i = 3;
        while (i <= n) {
            res.push_back(res[i - 3] + res[i - 2] + res[i - 1]);
            i++;
        }
        return res[n];
    }
};