fixed build ret

Author: xxx595588

data_structures/binary_tree/segment_tree_node.py

@@ -32,7 +32,7 @@ def build(self, start: int, end: int, nums: list[int]) -> Node:
         :return: Root node of the constructed subtree.
         """
         if start > end:
-            return
+            return Node(0, 0)
 
         if start == end:
             # Leaf node
@@ -156,4 +156,4 @@ def modify(
         if self.mode == "max":
             node.value = max(node.left.value, node.right.value)
         else:
-            node.value = node.left.value + node.right.value
+            node.value = node.left.value + node.right.value