1137. N-th Tribonacci Number

N-th Tribonacci Number - LeetCode

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.

  • code
class Solution {
    public int tribonacci(int n) {
        if (n == 0) return 0;
        if (n == 1 || n == 2) return 1;
        int before3 = 0, before2 = 1, before1 = 1;
        int updated = 0;
        for (int i = 3; i < n + 1; i++){
            updated =  before3 + before2 + before1;
            before3 = before2;
            before2 = before1;
            before1 = updated;
        }
        return updated;
    }
}