538. Convert BST to Greater Tree

• Difficulty: Easy

• Topics: Tree

• Similar Questions:

Problem:

Given 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 sum of all keys greater than the original key in BST.

Example:

Input: The root of a Binary Search Tree like this:
              5
            /   \
           2     13

Output: The root of a Greater Tree like this: 18 / \ 20 13 </pre> </p>

Solutions:

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* convertBST(TreeNode* root) {
        int sum = 0;
        helper(root, sum);
        return root;
    }

private:
    void helper(TreeNode* root, int& sum) {
        if (root == nullptr)    return;
        helper(root->right, sum);
        root->val += sum;
        sum = root->val;
        helper(root->left, sum);
    }
};