1137. N-th Tribonacci Number

Problem

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.

Solution (Java)

class Solution {
    public int tribonacci(int n) {
        if (n == 0) {
            return 0;
        } else if (n <= 2) {
            return 1;
        } else {
            int tn = 0;
            int tn1 = 1;
            int tn2 = 1;
            int tmp = 0;
            for (int i = 3; i <= n; i++) {
                tmp = tn + tn1 + tn2;
                tn = tn1;
                tn1 = tn2;
                tn2 = tmp;
            }
            return tmp;
        }
    }
}

Explain:

nope.

Complexity:

• Time complexity : O(n).
• Space complexity : O(n).