Added doctests in modular_exponential.py (#1775)

* added doctests in modular_exponential.py

* added doctests in modular_exponential.py

* added URL link

maths/modular_exponential.py

@@ -1,8 +1,20 @@
-"""Modular Exponential."""
+"""
+    Modular Exponential.
+    Modular exponentiation is a type of exponentiation performed over a modulus. 
+    For more explanation, please check https://en.wikipedia.org/wiki/Modular_exponentiation
+"""
 
+"""Calculate Modular Exponential."""
+def modular_exponential(base : int, power : int, mod : int):
+    """
+    >>> modular_exponential(5, 0, 10)
+    1
+    >>> modular_exponential(2, 8, 7)
+    4
+    >>> modular_exponential(3, -2, 9)
+    -1
+    """
 
-def modular_exponential(base, power, mod):
-    """Calculate Modular Exponential."""
     if power < 0:
         return -1
     base %= mod
@@ -13,6 +25,7 @@ def modular_exponential(base, power, mod):
             result = (result * base) % mod
         power = power >> 1
         base = (base * base) % mod
+
     return result
 
 
@@ -22,4 +35,8 @@ def main():
 
 
 if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
     main()