Update prefix_sum.py

Author: MaximSmolskiy

data_structures/arrays/prefix_sum.py

@@ -30,10 +30,22 @@ def get_sum(self, start: int, end: int) -> int:
         5
         >>> PrefixSum([1,2,3]).get_sum(2, 2)
         3
+        >>> PrefixSum([]).get_sum(0, 0)
+        Traceback (most recent call last):
+        ...
+        ValueError: The array is empty.
+        >>> PrefixSum([1,2,3]).get_sum(-1, 2)
+        Traceback (most recent call last):
+        ...
+        ValueError: Invalid range specified.
         >>> PrefixSum([1,2,3]).get_sum(2, 3)
         Traceback (most recent call last):
         ...
-        IndexError: list index out of range
+        ValueError: Invalid range specified.
+        >>> PrefixSum([1,2,3]).get_sum(2, 1)
+        Traceback (most recent call last):
+        ...
+        ValueError: Invalid range specified.
         """
         if not self.prefix_sum:
             raise ValueError("The array is empty.")