N-th Tribonacci Number in C++

Suppose we have a value n, we have to generate n-th Tribonacci number. The Tribonacci numbers are similar to the Fibonacci numbers, but here we are generating a term by adding three previous terms. Suppose we want to generate T(n), then the formula will be like below −

T(n) = T(n - 1) + T(n - 2) + T(n - 3)

The first few numbers to start, are {0, 1, 1}

We can solve them by following this algorithm −

Algorithm

• first := 0, second := 1, third := 1
• for i in range n – 3, do
   o next := first + second + third
   o first := second, second := third, third := next
• return third

Example (C++)

 Live Demo

#include<iostream>
using namespace std;
long tribonacci_gen(int n){
   //function to generate n tetranacci numbers
   int first = 0, second = 1, third = 1;
   for(int i = 0; i < n - 3; i++){
      int next = first + second + third;
      first = second;
      second = third;
      third = next;
   }
   return third;
}
main(){
   cout << "15th Tribonacci Term: " << tribonacci_gen(15);
}

Input

15

Output

15th Tribonacci Term: 1705