09. Convert BST to Greater Tree.cpp

/*
Convert BST to Greater Tree
===========================

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 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.
Note: This question is the same as 1038: https://leetcode.com/problems/binary-search-tree-to-greater-sum-tree/

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]

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

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

Constraints:
The number of nodes in the tree is in the range [0, 104].
-104 <= Node.val <= 104
All the values in the tree are unique.
root is guaranteed to be a valid binary search tree.
*/

/**
 * 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:
  void rev_inorder(TreeNode *node, int &sum)
  {
    if (!node)
      return;
    rev_inorder(node->right, sum);
    node->val += sum;
    sum = node->val;
    rev_inorder(node->left, sum);
  }

  TreeNode *convertBST(TreeNode *root)
  {
    int sum = 0;
    rev_inorder(root, sum);
    return root;
  }
};