In-place Convert BST into a Min-Heap

Given a Binary Search Tree, convert it into a Min-Heap containing the same elements in O(n) time. Do this in-place.

Input: Binary Search Tree
        8
     /    \
    4      12
  /  \     /  \
 2    6   10  14


Output - Min Heap
       2
     /    \
   4        6
 /  \     /   \
8    10  12   14
[Or any other tree that follows Min Heap
 properties and has same keys]

If we are allowed to use extra space, we can perform inorder traversal of the tree and store the keys in an auxiliary array. As we’re doing inorder traversal on a BST, array will be sorted. Finally, we construct a complete binary tree from the sorted array. We construct the binary tree level by level and from left to right by taking next minimum element from sorted array. The constructed binary tree will be a min-Heap. This solution works in O(n) time, but is not in-place.

How to do it in-place?
The idea is to convert the binary search tree into a sorted linked list first. We can do this by traversing the BST in inorder fashion. We add nodes at the beginning of current linked list and update head of the list using pointer to head pointer. Since we insert at the beginning, to maintain sorted order, we first traverse the right subtree before the left subtree. i.e. do a reverse inorder traversal.

Finally we convert the sorted linked list into a min-Heap by setting the left and right pointers appropriately. We can do this by doing a Level order traversal of the partially built Min-Heap Tree using queue and traversing the linked list at the same time. At every step, we take the parent node from queue, make next two nodes of linked list as children of the parent node, and enqueue the next two nodes to queue. As the linked list is sorted, the min-heap property is maintained.

Below is C++ implementation of above idea –

// Program to convert a BST into a Min-Heap
// in O(n) time and in-place
#include <iostream>
#include <queue>
using namespace std;

// Node for BST/Min-Heap
struct Node
{
    int data;
    Node *left, *right;
};

// Utility function for allocating node for BST
Node* newNode(int data)
{
    Node* node = new Node;
    node->data = data;
    node->left = node->right = NULL;
    return node;
}

// Utility function to print Min-heap level by level
void printLevelOrder(Node *root)
{
    // Base Case
    if (root == NULL)  return;

    // Create an empty queue for level order traversal
    queue<Node *> q;
    q.push(root);

    while (!q.empty())
    {
        int nodeCount = q.size();
        while (nodeCount > 0)
        {
            Node *node = q.front();
            cout << node->data << " ";
            q.pop();
            if (node->left)
                q.push(node->left);
            if (node->right)
                q.push(node->right);
            nodeCount--;
        }
        cout << endl;
    }
}

// A simple recursive function to convert a given
// Binary Search tree to Sorted Linked List
// root     --> Root of Binary Search Tree
// head_ref --> Pointer to head node of created
//              linked list
void BSTToSortedLL(Node* root, Node** head_ref)
{
    // Base cases
    if(root == NULL)
        return;

    // Recursively convert right subtree
    BSTToSortedLL(root->right, head_ref);

    // insert root into linked list
    root->right = *head_ref;

    // Change left pointer of previous head
    // to point to NULL
    if (*head_ref != NULL)
        (*head_ref)->left = NULL;

    // Change head of linked list
    *head_ref = root;

    // Recursively convert left subtree
    BSTToSortedLL(root->left, head_ref);
}

// Function to convert a sorted Linked
// List to Min-Heap.
// root --> Root of Min-Heap
// head --> Pointer to head node of sorted
//              linked list
void SortedLLToMinHeap(Node* &root, Node* head)
{
    // Base Case
    if (head == NULL)
        return;

    // queue to store the parent nodes
    queue<Node *> q;

    // The first node is always the root node
    root = head;

    // advance the pointer to the next node
    head = head->right;

    // set right child to NULL
    root->right = NULL;

    // add first node to the queue
    q.push(root);

    // run until the end of linked list is reached
    while (head)
    {
        // Take the parent node from the q and remove it from q
        Node* parent = q.front();
        q.pop();

        // Take next two nodes from the linked list and
        // Add them as children of the current parent node
        // Also in push them into the queue so that
        // they will be parents to the future nodes
        Node *leftChild = head;
        head = head->right;        // advance linked list to next node
        leftChild->right = NULL; // set its right child to NULL
        q.push(leftChild);

        // Assign the left child of parent
        parent->left = leftChild;

        if (head)
        {
            Node *rightChild = head;
            head = head->right; // advance linked list to next node
            rightChild->right = NULL; // set its right child to NULL
            q.push(rightChild);

            // Assign the right child of parent
            parent->right = rightChild;
        }
    }
}

// Function to convert BST into a Min-Heap
// without using any extra space
Node* BSTToMinHeap(Node* &root)
{
    // head of Linked List
    Node *head = NULL;

    // Convert a given BST to Sorted Linked List
    BSTToSortedLL(root, &head);

    // set root as NULL
    root = NULL;

    // Convert Sorted Linked List to Min-Heap
    SortedLLToMinHeap(root, head);
}

// Driver code
int main()
{
    /* Constructing below tree
                8
              /   \
             4     12
           /  \   /  \
          2    6 10   14
     */

    Node* root = newNode(8);
    root->left = newNode(4);
    root->right = newNode(12);
    root->right->left = newNode(10);
    root->right->right = newNode(14);
    root->left->left = newNode(2);
    root->left->right = newNode(6);

    BSTToMinHeap(root);

    /* Output - Min Heap
                2
              /   \
             4     6
           /  \   /  \
          8   10 12   14
     */

    printLevelOrder(root);

    return 0;
}

Output :

2 
4 6 
8 10 12 14 

This article is contributed by Aditya Goel. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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