BinarySearchTreeToGreaterSumTree.cpp

/*
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]
*/


/**
 * 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 reverseInorder(TreeNode *root, int &prev){
        if(!root) return;
        reverseInorder(root->right,prev);
        if(prev!=INT_MIN){
            int temp=prev;
            prev=root->val+prev;
            root->val+=temp;
        }else{
            prev=root->val;
        }
        reverseInorder(root->left,prev);
    }
    TreeNode* bstToGst(TreeNode* root) {
        int prev=INT_MIN;
        reverseInorder(root,prev);
    return root;
    }
};