1137. N-th Tribonacci Number

1. Question

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.

2. Examples

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

3. Constraints

• 0 <= n <= 37
• The answer is guaranteed to fit within a 32-bit integer, ie. answer <= 2^31 - 1.

4. References

来源：力扣（LeetCode） 链接：https://leetcode-cn.com/problems/n-th-tribonacci-number 著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。

5. Solutions

class Solution {
    public int tribonacci(int n) {
        int[] arr = {0, 1, 1, 0};
        if (n <= 2) {
            return arr[n];
        }
        for (int i = 2; i < n; i++) {
            arr[3] = arr[0] + arr[1] + arr[2];
            arr[0] = arr[1];
            arr[1] = arr[2];
            arr[2] = arr[3];
        }
        return arr[3];
    }
}

class Solution {

  public int tribonacci(int n) {
    int a = 0;
    int b = 1;
    int c = 1;
    int d = 0;

    if (n < 2) {
      return n;
    }

    if (n == 2) {
      return 1;
    }

    for (int i = 3; i <= n; i++) {
      d = a + b + c;
      a = b;
      b = c;
      c = d;
    }

    return d;

  }
}