Convert BST To Greater Tree Problem


Description

LeetCode Problem 538.

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:

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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:

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

Example 3:

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Input: root = [1,0,2]
Output: [3,3,2]

Example 4:

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Input: root = [3,2,4,1]
Output: [7,9,4,10]

Constraints:

  • The number of nodes in the tree is in the range [0, 10^4].
  • -10^4 <= Node.val <= 10^4
  • All the values in the tree are unique.
  • root is guaranteed to be a valid binary search tree.


Sample C++ Code

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/**
 * 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:
    int sum = 0;
    TreeNode* convertBST(TreeNode* root) {
        if (root != NULL) {
            convertBST(root->right);
            sum += root->val;
            root->val = sum;
            convertBST(root->left);
        }
        return root;
    }
};




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