# C Programming – Convert a BST to a Binary Tree such that sum of all greater keys is added to every key

Convert a BST to a Binary Tree such that sum of all greater keys is added to every key - Given a Binary Search Tree (BST), convert it to a Binary Tree.

Given a Binary Search Tree (BST), convert it to a Binary Tree such that every key of the original BST is changed to key plus sum of all greater keys in BST.

Examples:

```Input: Root of following BST
5
/   \
2     13

Output: The given BST is converted to following Binary Tree
18
/   \
20     13
```

Source: Convert a BST

Solution: Do reverse Inoorder traversal. Keep track of the sum of nodes visited so far. Let this sum be sum. For every node currently being visited, first add the key of this node to sum, i.e. sum = sum + node->key. Then change the key of current node to sum, i.e., node->key = sum.
When a BST is being traversed in reverse Inorder, for every key currently being visited, all keys that are already visited are all greater keys.

C Program
``````// Program to change a BST to Binary Tree such that key of a node becomes
// original key plus sum of all greater keys in BST
#include <stdio.h>
#include <stdlib.h>

/* A BST node has key, left child and right child */
struct node
{
int key;
struct node* left;
struct node* right;
};

/* Helper function that allocates a new node with the given key and
NULL left and right  pointers.*/
struct node* newNode(int key)
{
struct node* node = (struct node*)malloc(sizeof(struct node));
node->key = key;
node->left = NULL;
node->right = NULL;
return (node);
}

// A recursive function that traverses the given BST in reverse inorder and
// for every key, adds all greater keys to it
void addGreaterUtil(struct node *root, int *sum_ptr)
{
// Base Case
if (root == NULL)
return;

// Recur for right subtree first so that sum of all greater
// nodes is stored at sum_ptr

// Update the value at sum_ptr
*sum_ptr = *sum_ptr + root->key;

// Update key of this node
root->key = *sum_ptr;

// Recur for left subtree so that the updated sum is added
// to smaller nodes
}

// A wrapper over addGreaterUtil().  It initializes sum and calls
// addGreaterUtil() to recursivel upodate and use value of sum
{
int sum = 0;
}

// A utility function to print inorder traversal of Binary Tree
void printInorder(struct node* node)
{
if (node == NULL)
return;
printInorder(node->left);
printf("%d ", node->key);
printInorder(node->right);
}

// Driver program to test above function
int main()
{
/* Create following BST
5
/   \
2     13  */
node *root = newNode(5);
root->left = newNode(2);
root->right = newNode(13);

printf(" Inorder traversal of the given tree\n");
printInorder(root);

printf("\n Inorder traversal of the modified tree\n");
printInorder(root);

return 0;
}`````` 