N-th Tribonacci Number

The tribonacci sequence is defined as T0 = 0, T1 = 1, T2 = 1, and for n >= 3, Tn = T(n-1) + T(n-2) + T(n-3). It extends the Fibonacci concept to three preceding terms. Given n, return the value of Tn.

Examples
Input: 4
Output: 4
Hints

N-th Tribonacci Number

The tribonacci sequence is defined as T0 = 0, T1 = 1, T2 = 1, and for n >= 3, Tn = T(n-1) + T(n-2) + T(n-3). It extends the Fibonacci concept to three preceding terms. Given n, return the value of Tn.