N-th Tribonacci Number

Statement

Given a number n, calculate the corresponding Tribonacci number. The Tribonacci sequence $T_n$ is defined as:

$T_0 = 0,\space T_1 = 1,\space T_2 = 1$, and $\space T_{n+3} = T_n + T_{n+1} + T_{n+2},\space$ for $n >= 0$

The input number, n, is a non-negative integer.

Constraints:

• $0 \leq$ n $\leq 37$
• The answer is guaranteed to fit within a 32-bit integer, i.e., answer $\leq 2 ^{31} - 1$

Examples