1137. N-th Tribonacci Number.java

/*
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:
class Solution {
    public int tribonacci(int n) {
        if(n == 0) return 0;
        if(n == 1) return 1;
        if(n == 2) return 1;
        
        int num0 = 0,num1 = 1,num2=1;
        int sum = 0;
        for(int i = 3; i<=n;i++){
            sum = num0 + num1 + num2;
            num0 = num1;
            num1 = num2;
            num2 = sum;
        }
        
        return sum;
    }
}