From 210e9a2d6e678af590843ff02cfef113f0d4c3e8 Mon Sep 17 00:00:00 2001
From: Putul Singh <127419636+putul03@users.noreply.github.com>
Date: Sat, 19 Oct 2024 16:57:21 +0530
Subject: [PATCH] Update persistent_segment_tree.py

---
 data_structures/persistent_segment_tree.py | 58 +++++++++++++---------
 1 file changed, 34 insertions(+), 24 deletions(-)

diff --git a/data_structures/persistent_segment_tree.py b/data_structures/persistent_segment_tree.py
index cf6dbed0d..074e137bb 100644
--- a/data_structures/persistent_segment_tree.py
+++ b/data_structures/persistent_segment_tree.py
@@ -1,13 +1,11 @@
 from __future__ import annotations
 
-
 class Node:
     def __init__(self, value: int = 0) -> None:
         self.value: int = value
         self.left: Node | None = None
         self.right: Node | None = None
 
-
 class PersistentSegmentTree:
     def __init__(self, arr: list[int]) -> None:
         self.n: int = len(arr)
@@ -54,12 +52,22 @@ class PersistentSegmentTree:
         self.roots.append(new_root)
         return len(self.roots) - 1
 
-    def _update(
-        self, node: Node | None, start: int, end: int, index: int, value: int
-    ) -> Node:
-        if node is None:
-            raise ValueError("Cannot update a None node")
+    def _update(self, node: Node, start: int, end: int, index: int, value: int) -> Node:
+        """
+        Updates the node for the specified index and value and returns the new node.
 
+        >>> pst = PersistentSegmentTree([1, 2, 3, 4])
+        >>> old_root = pst.roots[0]
+        >>> new_root = pst._update(old_root, 0, 3, 1, 5)  # Update index 1 to 5
+        >>> new_root.value  # New sum after update
+        13
+        >>> old_root.value  # Old root remains unchanged
+        10
+        >>> new_root.left.value  # Updated left child
+        6
+        >>> new_root.right.value  # Right child remains the same
+        7
+        """
         if start == end:
             return Node(value)
 
@@ -68,14 +76,12 @@ class PersistentSegmentTree:
 
         if index <= mid:
             new_node.left = self._update(node.left, start, mid, index, value)
-            new_node.right = node.right
+            new_node.right = node.right  # Ensure right node is the same as the original
         else:
-            new_node.left = node.left
+            new_node.left = node.left  # Ensure left node is the same as the original
             new_node.right = self._update(node.right, mid + 1, end, index, value)
 
-        new_node.value = new_node.left.value + (
-            new_node.right.value if new_node.right else 0
-        )
+        new_node.value = new_node.left.value + (new_node.right.value if new_node.right else 0)
 
         return new_node
 
@@ -91,31 +97,35 @@ class PersistentSegmentTree:
         >>> version_1 = pst.update(0, 1, 5)  # Update index 1 to 5
         >>> pst.query(version_1, 0, 3)  # Sum of all elements in new version
         13
-        >>> pst.query(version_1, 1, 2)  # Sum of elements at index 1 and 2 in new version
+        >>> pst.query(version_1, 1, 2)  # Sum of elements at index 1 and 2 
         8
         """
         return self._query(self.roots[version], 0, self.n - 1, left, right)
 
-    def _query(
-        self, node: Node | None, start: int, end: int, left: int, right: int
-    ) -> int:
-        if node is None:
-            return 0
+    def _query(self, node: Node, start: int, end: int, left: int, right: int) -> int:
+        """
+        Queries the sum of values in the range [left, right] for the given node.
 
-        if left > end or right < start:
+        >>> pst = PersistentSegmentTree([1, 2, 3, 4])
+        >>> root = pst.roots[0]
+        >>> pst._query(root, 0, 3, 1, 2)  # Sum of elements at index 1 and 2
+        5
+        >>> pst._query(root, 0, 3, 0, 3)  # Sum of all elements
+        10
+        >>> pst._query(root, 0, 3, 2, 3)  # Sum of elements at index 2 and 3
+        7
+        """
+        if node is None or left > end or right < start:
             return 0
         if left <= start and right >= end:
             return node.value
         mid = (start + end) // 2
-        return self._query(node.left, start, mid, left, right) + self._query(
-            node.right, mid + 1, end, left, right
-        )
-
+        return (self._query(node.left, start, mid, left, right) +
+                self._query(node.right, mid + 1, end, left, right))
 
 # Running the doctests
 if __name__ == "__main__":
     import doctest
-
     print("Running doctests...")
     result = doctest.testmod()
     print(f"Ran {result.attempted} tests, {result.failed} failed.")