[LeetCode Sync] Runtime - 4 ms (39.70%), Memory - 19.8 MB (63.87%)

Author: NamVr

0538-convert-bst-to-greater-tree/README.md

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+<p>Given the <code>root</code> of a Binary Search Tree (BST), convert it to a Greater Tree such that every key of the original BST is changed to the original key plus the sum of all keys greater than the original key in BST.</p>
+
+<p>As a reminder, a <em>binary search tree</em> is a tree that satisfies these constraints:</p>
+
+<ul>
+	<li>The left subtree of a node contains only nodes with keys <strong>less than</strong> the node&#39;s key.</li>
+	<li>The right subtree of a node contains only nodes with keys <strong>greater than</strong> the node&#39;s key.</li>
+	<li>Both the left and right subtrees must also be binary search trees.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2019/05/02/tree.png" style="width: 500px; height: 341px;" />
+<pre>
+<strong>Input:</strong> root = [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]
+<strong>Output:</strong> [30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [0,null,1]
+<strong>Output:</strong> [1,null,1]
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[0, 10<sup>4</sup>]</code>.</li>
+	<li><code>-10<sup>4</sup> &lt;= Node.val &lt;= 10<sup>4</sup></code></li>
+	<li>All the values in the tree are <strong>unique</strong>.</li>
+	<li><code>root</code> is guaranteed to be a valid binary search tree.</li>
+</ul>
+
+<p>&nbsp;</p>
+<p><strong>Note:</strong> This question is the same as 1038: <a href="https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/" target="_blank">https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/</a></p>

0538-convert-bst-to-greater-tree/solution.py

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+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:
+        # create a variable to track the maximum sum that will be assigned to the last node
+        sum = 0
+
+        # we will start with the root
+        def helper(root):
+            nonlocal sum
+
+            # if root is not null, run the loop at the greater side (right)
+            # now sum will be updated with right's sum
+            # append the value to the root.
+
+            # after that get the current sum and traverse to the left
+            if root:
+                helper(root.right)
+                root.val += sum
+
+                sum = root.val
+                helper(root.left)
+            
+            return root
+
+        return helper(root)