day 25 q2 solution in python and cpp

Day-25/Q2: Convert BST to Greater Tree/Tech-neophyte--pc.md

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+## Approach 1: Using stack
+## Python code:
+```
+class Solution:
+    def convertBST(self, root: Optional[TreeNode]) -> Optional[TreeNode]:        
+        stack, summ, node = [], 0, root
+        while stack or node:
+            while node:
+                stack.append(node)
+                node = node.right
+            node = stack.pop()
+            node.val += summ
+            summ = node.val
+            node = node.left
+        return root
+```
+## Approach 2: using recursive dfs
+## python code:
+```
+def dfs(node, val):
+            if node.right:
+                val = dfs(node.right, val)
+            val += node.val
+            node.val = val
+            if node.left:
+                val = dfs(node.left, val)
+            return val
+        dfs(root, 0)
+        return root
+```
+## Approach
+This C++ code defines a `Solution` class with a method `convertBST` that takes the root of a binary search tree (BST) and returns the same tree modified to be a Greater Tree. The `convertToGT` function recursively traverses the BST in reverse in-order, updating node values to the sum of greater values encountered so far. The final modified tree is returned.
+## cpp Code
+```
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
+ *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
+ * };
+ */
+class Solution {
+public:
+    TreeNode* convertToGT(TreeNode* root,int& sum){
+        if(!root)return NULL;
+        convertToGT(root->right,sum);
+        sum+=root->val;
+        root->val=sum;
+        convertToGT(root->left,sum);
+        return root;
+    }
+    TreeNode* convertBST(TreeNode* root) {
+        int sum=0;
+        return convertToGT(root,sum);
+    }
+
+};
+```