add solution for tribonacci sequence challenge

solutions/the_tribonacci_sequence/main.py

@@ -0,0 +1,24 @@
+def tribonacci(n: int) -> int:
+    """
+        Calculate the nth tribonacci number.
+
+        Args:
+            n (int): Index of the tribonacci number to calculate.
+
+        Returns:
+            int: The nth tribonacci number.
+
+        Created on 1/01/2025
+    Author: Ana Isabel Murillo
+    """
+    if n == 0:
+        return 0
+    elif n == 1 or n == 2:
+        return 1
+
+    a, b, c = 0, 1, 1
+
+    for _ in range(3, n + 1):
+        a, b, c = b, c, a + b + c
+
+    return c