leetcode-1137-easy

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.

思路一： 斐波那契数列的变种，思路按照斐波那契的做法

public int tribonacci(int n) {
    int[] result = new int[Math.max(n+1, 4)];
    result[0] = 0;
    result[1] = 1;
    result[2] = 1;

    for (int i = 3; i < n+1; i++) {
        result[i] = result[i - 1] + result[i - 2] + result[i - 3];
    }

    return result[n];
}