Convert BST to Greater Tree in C++

Suppose we have a Binary Search Tree, we have to convert it into a Greater Tree such that every key of the original BST is changed to the original key + sum of all keys greater than the original key in BST.

So, if the input is like

then the output will be

To solve this, we will follow these steps −

  • Define a function revInorder(), this will take tree root and s,

  • if root is null, then −

    • return

  • revInorder(right of root, s)

  • s := s + val of root

  • val of root := s

  • revInorder(left of root, s)

  • From the main method, do the following −

  • if root is null, then −

    • return null

  • sum := 0

  • revInorder(root, sum)

  • return root

Example 

Let us see the following implementation to get a better understanding −

 Live Demo

#include <bits/stdc++.h>
using namespace std;
class TreeNode{
   public:
      int val;
      TreeNode *left, *right;
      TreeNode(int data){
         val = data;
         left = NULL;
         right = NULL;
      }
};
void insert(TreeNode **root, int val){
   queue<TreeNode*> q;
   q.push(*root);
   while(q.size()){
      TreeNode *temp = q.front();
      q.pop();
      if(!temp->left){
         if(val != NULL)
            temp->left = new TreeNode(val);
         else
            temp->left = new TreeNode(0);
         return;
      }
      else{
         q.push(temp->left);
      }
      if(!temp->right){
         if(val != NULL)
            temp->right = new TreeNode(val);
         else
            temp->right = new TreeNode(0);
         return;
      }
      else{
         q.push(temp->right);
      }
   }
}
TreeNode *make_tree(vector<int> v){
   TreeNode *root = new TreeNode(v[0]);
   for(int i = 1; i<v.size(); i++){
      insert(&root, v[i]);
   }
   return root;
}
void tree_level_trav(TreeNode*root){
   if (root == NULL) return;
      cout << "[";
   queue<TreeNode *> q;
   TreeNode *curr;
   q.push(root);
   q.push(NULL);
   while (q.size() > 1) {
      curr = q.front();
      q.pop();
      if (curr == NULL){
         q.push(NULL);
      }
      else {
         if(curr->left)
            q.push(curr->left);
         if(curr->right)
            q.push(curr->right);
         if(curr == NULL || curr->val == 0){
            cout << "null" << ", ";
         }
         else{
            cout << curr->val << ", ";
         }
      }
   }
   cout << "]"<<endl;
}
class Solution {
   public:
   void revInorder(TreeNode *root,int &s){
      if (root == NULL || root->val == 0)
         return;
      revInorder(root->right, s);
      s += root->val;
      root->val = s;
      revInorder(root->left, s);
   }
   TreeNode* convertBST(TreeNode* root){
      if (root == NULL || root->val == 0)
         return NULL;
      int sum = 0;
      revInorder(root, sum);
      return root;
   }
};
main(){
   Solution ob;
   vector<int> v = {5,2,8,NULL,NULL,6,9};
   TreeNode *root = make_tree(v);
   tree_level_trav(ob.convertBST(root));
}

Input

{5,2,8,NULL,NULL,6,9}

Output

[28, 30, 17, null, null, 23, 9, ]

Updated on: 10-Jun-2020

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