Algorithm for tribonacci number calculation added

Author: megha-66

maths/tribonacci.py

@@ -0,0 +1,21 @@
+"""
+This program calculates the "Nth Tribonacci number in a series. 
+
+The Tribonacci sequence Tn is defined as follows :- 
+
+T(0) = 0; T(1) = 1; T(2) = 1; and T(n+3) = T(n) + T(n+1) + T(n+3) for n>=0 
+
+In this program, we assume an integer 'n' is given and we have to calculate nth Tribinacci number 
+
+"""
+
+def tribonacci(self, n : int) -> int :
+    trib = [0,1,1]
+    for i in range(3,n+1):
+        x = trib[i-1] + trib[i-2] + trib[i-3]
+        trib.append(x)
+    return trib[n]
+
+
+if __name__ == "__main__" :
+    print(tribonacci(25)) #prints 1389537