Last updated on January 5th, 2025 at 11:55 pm

Here, we see a Convert BST to Greater Tree LeetCode Solution. This Leetcode problem is solved using different approaches in many programming languages, such as C++, Java, JavaScript, Python, etc.

List of all LeetCode Solution

Topics

Tree

Companies

Amazon

Level of Question

Medium

Convert BST to Greater Tree LeetCode Solution

1. Problem Statement

Given the root 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.

As a reminder, a binary search tree is a tree that satisfies these constraints:

• The left subtree of a node contains only nodes with keys less than the node’s key.
• The right subtree of a node contains only nodes with keys greater than the node’s key.
• Both the left and right subtrees must also be binary search trees.

Example 1:

Input: root = [4,1,6,0,2,5,7,null,null,null,3,null,null,null,8]
Output: [30,36,21,36,35,26,15,null,null,null,33,null,null,null,8]

Example 2:
Input: root = [0,null,1]
Output: [1,null,1]

2. Coding Pattern Used in Solution

The coding pattern used in the provided code is “Tree Depth First Search (DFS)”. Specifically, it uses a Reverse Inorder Traversal (right -> root -> left) to traverse the binary search tree (BST). This traversal ensures that nodes are visited in descending order of their values, which is essential for converting the BST into a Greater Tree.

3. Code Implementation in Different Languages

3.1 Convert BST to Greater Tree C++

class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
        int num = 0;
        solve(root, num);
        return root;        
    }

    void solve(TreeNode *root, int &num)
    {
        if(root == NULL)
            return;
        solve(root->right, num);
        root->val = num + root->val;
        num = root->val;
        solve(root->left, num);
    }
};

3.2 Convert BST to Greater Tree Java

class Solution {
    int sum = 0;
    public TreeNode convertBST(TreeNode root) {
        if(root==null){
            return root;
        }
        reverseInorder(root);
        return root;
    }
    private void reverseInorder(TreeNode root){
        if(root==null){
            return;
        }
        reverseInorder(root.right);
        root.val = root.val + sum;
        sum = root.val;
        reverseInorder(root.left);
        return;
    }
}

3.3 Convert BST to Greater Tree JavaScript

var convertBST = function(root) {
    let sum = 0, prev = 0;
    function rec(root){
        if(root){
            rec(root.right);
            sum += root.val;
            root.val += prev;
            prev = sum;
            rec(root.left);
        }
    }
    rec(root);
    return root;
}

3.4 Convert BST to Greater Tree Python

class Solution(object):
    def convertBST(self, root):
        def dfs(node):
            if node:
                dfs(node.right)
                node.val += dfs.greater_sum
                dfs.greater_sum = node.val
                dfs(node.left)
        dfs.greater_sum = 0
        dfs(root)
        return root

4. Time and Space Complexity

	Time Complexity	Space Complexity
C++	O(n)	O(h)
Java	O(n)	O(h)
JavaScript	O(n)	O(h)
Python	O(n)	O(h)

where, h is the height of the tree.