Algorithm for Nth Tribinacci Number calculation

maths/tribonacci.py

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+"""
+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(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__" :
+    import doctest
+    doctest.testmod()