Tribonacci Numbers

The tribonacci numbers start 0, 1, 1, and every number after that is the sum of the three numbers right before it: 0, 1, 1, 2, 4, 7, 13, 24, 44, and so on.

Write tribonacci(n) that returns the number at position n, counting positions from 0. So tribonacci(0) is 0, tribonacci(1) and tribonacci(2) are both 1, and tribonacci(3) is 0 + 1 + 1 = 2.

Examples

tribonacci(4)   # returns 4
tribonacci(7)   # returns 24
tribonacci(10)  # returns 149

Constraints

• 0 <= n <= 100

Note

Positions go up to 100, so your function has to be quick for big positions too. Python integers have no size limit, so the answer is always exact.