1038

1038. Binary Search Tree to Greater Sum Tree (Medium)

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]

 

Constraints:

• The number of nodes in the tree is in the range [1, 100].
• 0 <= Node.val <= 100
• All the values in the tree are unique.

 

Note: This question is the same as 538: https://leetcode.com/problems/convert-bst-to-greater-tree/

Companies:
Facebook, Amazon

Related Topics:
Tree, Depth-First Search, Binary Search Tree, Binary Tree

Solution 1.

// OJ: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(H)
class Solution {
    int sum = 0;
public:
    TreeNode* bstToGst(TreeNode* root) {
        if (!root) return nullptr;
        bstToGst(root->right);
        root->val = (sum += root->val);
        bstToGst(root->left);
        return root;
    }
};