n_th_tribonacci_number.dart

/*




-* 1137. 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.


*/

class Solution {
  int tribonacci(int n) {
    if (n == 0) return 0;
    if (n == 1 || n == 2) return 1;
    List<int> Tribonacci = List.filled(n + 1, 0);
    Tribonacci[0] = 0;
    Tribonacci[1] = 1;
    Tribonacci[2] = 1;
    for (int i = 3; i < n + 1; i++) {
      Tribonacci[i] = Tribonacci[i - 1] + Tribonacci[i - 2] + Tribonacci[i - 3];
    }
    return Tribonacci[n];
  }
}

class B {
  int tribonacci(int n) {
    if (n == 0) return 0;
    if (n == 1) return 1;
    if (n == 2) return 1;
    int a = 0, b = 1, c = 1, d = 0;
    for (int i = 3; i <= n; i++) {
      d = a + b + c;
      a = b;
      b = c;
      c = d;
    }
    return d;
  }
}

class C {
  int tribonacci(int n) {
    if (n < 2) return n;
    if (n == 2) return 1;
    return tribonacci(n - 1) + tribonacci(n - 2) + tribonacci(n - 3);
  }
}