LeetCode 1038: Binary Search Tree to Greater Sum Tree Solution

Master LeetCode problem 1038 (Binary Search Tree to Greater Sum Tree), a medium challenge, with our optimized solutions in Java, C++, and Python. Explore detailed explanations, test your code in our interactive editor, and prepare for coding interviews.

1038. Binary Search Tree to Greater Sum Tree

Medium

Tree Depth-First Search Binary Search Tree Binary Tree

Problem Explanation

Explanation

To convert a Binary Search Tree (BST) to a Greater Tree, we can perform a reverse inorder traversal of the tree. In this traversal, we visit the nodes in descending order (right-root-left), updating a running sum of the nodes visited so far. The key idea is to keep track of the sum of greater values while traversing the tree and update each node's value with this sum.

Algorithm:

Initialize a variable sum to store the running sum of nodes visited so far.
Perform a reverse inorder traversal of the BST:
- Recursively visit the right subtree.
- Update the current node's value with the sum and update sum with the current node's value.
- Recursively visit the left subtree.
Return the modified BST root.

Time Complexity: O(n) - where n is the number of nodes in the BST. Space Complexity: O(h) - where h is the height of the BST.

Solution Code

class Solution {
    int sum = 0;

    public TreeNode bstToGst(TreeNode root) {
        if (root != null) {
            bstToGst(root.right);
            sum += root.val;
            root.val = sum;
            bstToGst(root.left);
        }
        return root;
    }
}

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