K’th Largest element in BST using constant extra space

• Difficulty Level : Hard
• Last Updated : 21 May, 2021

Given a binary search tree, task is to find Kth largest element in the binary search tree.
Example:

```Input :  k = 3
Root of following BST
10
/    \
4      20
/      /   \
2     15     40
Output : 15```

Attention reader! Don’t stop learning now. Get hold of all the important DSA concepts with the DSA Self Paced Course at a student-friendly price and become industry ready.  To complete your preparation from learning a language to DS Algo and many more,  please refer Complete Interview Preparation Course.

In case you wish to attend live classes with experts, please refer DSA Live Classes for Working Professionals and Competitive Programming Live for Students.

The idea is to use Reverse Morris Traversal which is based on Threaded Binary Trees. Threaded binary trees use the NULL pointers to store the successor and predecessor information which helps us to utilize the wasted memory by those NULL pointers.
The special thing about Morris traversal is that we can do Inorder traversal without using stack or recursion which saves us memory consumed by stack or recursion call stack.
Reverse Morris traversal is just the reverse of Morris traversal which is majorly used to do Reverse Inorder traversal with constant O(1) extra memory consumed as it does not uses any Stack or Recursion.
To find Kth largest element in a Binary search tree, the simplest logic is to do reverse inorder traversal and while doing reverse inorder traversal simply keep a count of number of Nodes visited. When the count becomes equal to k, we stop the traversal and print the data. It uses the fact that reverse inorder traversal will give us a list sorted in descending order.

Algorithm

```1) Initialize Current as root.
2) Initialize a count variable to 0.
3) While current is not NULL :
3.1) If current has no right child
a) Increment count and check if count is equal to K.
1) If count is equal to K, simply return current
Node as it is the Kth largest Node.
b) Otherwise, Move to the left child of current.

3.2) Else, here we have 2 cases:
a) Find the inorder successor of current Node.
Inorder successor is the left most Node
in the right subtree or right child itself.
b) If the left child of the inorder successor is NULL:
1) Set current as the left child of its inorder
successor.
2) Move current Node to its right.
c) Else, if the threaded link between the current Node
and it's inorder successor already exists :
1) Set left pointer of the inorder successor as NULL.
2) Increment count and check if count is equal to K.
a) If count is equal to K, simply return current
Node as it is the Kth largest Node.

3) Otherwise, Move current to it's left child.```

C++

 `// CPP code for finding K-th largest Node using O(1)``// extra memory and reverse Morris traversal.``#include ``using` `namespace` `std;` `struct` `Node {``    ``int` `data;``    ``struct` `Node *left, *right;``};` `// helper function to create a new Node``Node* newNode(``int` `data)``{``    ``Node* temp = ``new` `Node;``    ``temp->data = data;``    ``temp->right = temp->left = NULL;``    ``return` `temp;``}` `Node* KthLargestUsingMorrisTraversal(Node* root, ``int` `k)``{``    ``Node* curr = root;``    ``Node* Klargest = NULL;` `    ``// count variable to keep count of visited Nodes``    ``int` `count = 0;` `    ``while` `(curr != NULL) {``        ``// if right child is NULL``        ``if` `(curr->right == NULL) {` `            ``// first increment count and check if count = k``            ``if` `(++count == k)``                ``Klargest = curr;` `            ``// otherwise move to the left child``            ``curr = curr->left;``        ``}` `        ``else` `{` `            ``// find inorder successor of current Node``            ``Node* succ = curr->right;` `            ``while` `(succ->left != NULL && succ->left != curr)``                ``succ = succ->left;` `            ``if` `(succ->left == NULL) {` `                ``// set left child of successor to the``                ``// current Node``                ``succ->left = curr;` `                ``// move current to its right``                ``curr = curr->right;``            ``}` `            ``// restoring the tree back to original binary``            ``//  search tree removing threaded links``            ``else` `{` `                ``succ->left = NULL;` `                ``if` `(++count == k)``                    ``Klargest = curr;` `                ``// move current to its left child``                ``curr = curr->left;``            ``}``        ``}``    ``}` `    ``return` `Klargest;``}` `int` `main()``{``    ``// Your C++ Code``    ``/* Constructed binary tree is``          ``4``        ``/   \``       ``2     7``     ``/  \   /  \``    ``1    3  6    10 */` `    ``Node* root = newNode(4);``    ``root->left = newNode(2);``    ``root->right = newNode(7);``    ``root->left->left = newNode(1);``    ``root->left->right = newNode(3);``    ``root->right->left = newNode(6);``    ``root->right->right = newNode(10);` `    ``cout << ``"Finding K-th largest Node in BST : "``         ``<< KthLargestUsingMorrisTraversal(root, 2)->data;` `    ``return` `0;``}`

Java

 `// Java Program for finding K-th largest Node using O(1)``// extra memory and reverse Morris traversal.``class` `GfG``{` `static` `class` `Node``{``    ``int` `data;``    ``Node left, right;``}` `// helper function to create a new Node``static` `Node newNode(``int` `data)``{``    ``Node temp = ``new` `Node();``    ``temp.data = data;``    ``temp.right = ``null``;``    ``temp.left = ``null``;``    ``return` `temp;``}` `static` `Node KthLargestUsingMorrisTraversal(Node root, ``int` `k)``{``    ``Node curr = root;``    ``Node Klargest = ``null``;` `    ``// count variable to keep count of visited Nodes``    ``int` `count = ``0``;` `    ``while` `(curr != ``null``)``    ``{``        ``// if right child is NULL``        ``if` `(curr.right == ``null``)``        ``{` `            ``// first increment count and check if count = k``            ``if` `(++count == k)``                ``Klargest = curr;` `            ``// otherwise move to the left child``            ``curr = curr.left;``        ``}` `        ``else``        ``{` `            ``// find inorder successor of current Node``            ``Node succ = curr.right;` `            ``while` `(succ.left != ``null` `&& succ.left != curr)``                ``succ = succ.left;` `            ``if` `(succ.left == ``null``)``            ``{` `                ``// set left child of successor to the``                ``// current Node``                ``succ.left = curr;` `                ``// move current to its right``                ``curr = curr.right;``            ``}` `            ``// restoring the tree back to original binary``            ``// search tree removing threaded links``            ``else``            ``{` `                ``succ.left = ``null``;` `                ``if` `(++count == k)``                    ``Klargest = curr;` `                ``// move current to its left child``                ``curr = curr.left;``            ``}``        ``}``    ``}``    ``return` `Klargest;``}` `// Driver code``public` `static` `void` `main(String[] args)``{``    ``// Your Java Code``    ``/* Constructed binary tree is``        ``4``        ``/ \``    ``2 7``    ``/ \ / \``    ``1 3 6 10 */` `    ``Node root = newNode(``4``);``    ``root.left = newNode(``2``);``    ``root.right = newNode(``7``);``    ``root.left.left = newNode(``1``);``    ``root.left.right = newNode(``3``);``    ``root.right.left = newNode(``6``);``    ``root.right.right = newNode(``10``);` `    ``System.out.println(``"Finding K-th largest Node in BST : "` `+``                    ``KthLargestUsingMorrisTraversal(root, ``2``).data);``}``}`

Python3

 `# Python3 code for finding K-th largest``# Node using O(1) extra memory and``# reverse Morris traversal.` `# helper function to create a new Node``class` `newNode:``    ``def` `__init__(``self``, data):``        ``self``.data ``=` `data``        ``self``.right ``=` `self``.left ``=` `None` `def` `KthLargestUsingMorrisTraversal(root, k):``    ``curr ``=` `root``    ``Klargest ``=` `None` `    ``# count variable to keep count``    ``# of visited Nodes``    ``count ``=` `0` `    ``while` `(curr !``=` `None``):``        ` `        ``# if right child is None``        ``if` `(curr.right ``=``=` `None``):` `            ``# first increment count and``            ``# check if count = k``            ``count ``+``=` `1``            ``if` `(count ``=``=` `k):``                ``Klargest ``=` `curr` `            ``# otherwise move to the left child``            ``curr ``=` `curr.left` `        ``else``:` `            ``# find inorder successor of``            ``# current Node``            ``succ ``=` `curr.right` `            ``while` `(succ.left !``=` `None` `and``                   ``succ.left !``=` `curr):``                ``succ ``=` `succ.left` `            ``if` `(succ.left ``=``=` `None``):` `                ``# set left child of successor``                ``# to the current Node``                ``succ.left ``=` `curr` `                ``# move current to its right``                ``curr ``=` `curr.right` `            ``# restoring the tree back to ``            ``# original binary search tree``            ``# removing threaded links``            ``else``:` `                ``succ.left ``=` `None``                ``count ``+``=` `1``                ``if` `(count ``=``=` `k):``                    ``Klargest ``=` `curr` `                ``# move current to its left child``                ``curr ``=` `curr.left` `    ``return` `Klargest` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:` `    ``# Constructed binary tree is``    ``#     4``    ``#     / \``    ``# 2     7``    ``# / \ / \``    ``# 1 3 6 10``    ``root ``=` `newNode(``4``)``    ``root.left ``=` `newNode(``2``)``    ``root.right ``=` `newNode(``7``)``    ``root.left.left ``=` `newNode(``1``)``    ``root.left.right ``=` `newNode(``3``)``    ``root.right.left ``=` `newNode(``6``)``    ``root.right.right ``=` `newNode(``10``)` `    ``print``(``"Finding K-th largest Node in BST : "``,``           ``KthLargestUsingMorrisTraversal(root, ``2``).data)` `# This code is contributed by PranchalK`

C#

 `// C# Program for finding K-th largest Node using O(1)``// extra memory and reverse Morris traversal.``using` `System;``using` `System.Collections.Generic;` `class` `GfG``{` `public` `class` `Node``{``    ``public` `int` `data;``    ``public` `Node left, right;``}` `// helper function to create a new Node``static` `Node newNode(``int` `data)``{``    ``Node temp = ``new` `Node();``    ``temp.data = data;``    ``temp.right = ``null``;``    ``temp.left = ``null``;``    ``return` `temp;``}` `static` `Node KthLargestUsingMorrisTraversal(Node root, ``int` `k)``{``    ``Node curr = root;``    ``Node Klargest = ``null``;` `    ``// count variable to keep count of visited Nodes``    ``int` `count = 0;` `    ``while` `(curr != ``null``)``    ``{``        ``// if right child is NULL``        ``if` `(curr.right == ``null``)``        ``{` `            ``// first increment count and check if count = k``            ``if` `(++count == k)``                ``Klargest = curr;` `            ``// otherwise move to the left child``            ``curr = curr.left;``        ``}` `        ``else``        ``{` `            ``// find inorder successor of current Node``            ``Node succ = curr.right;` `            ``while` `(succ.left != ``null` `&& succ.left != curr)``                ``succ = succ.left;` `            ``if` `(succ.left == ``null``)``            ``{` `                ``// set left child of successor to the``                ``// current Node``                ``succ.left = curr;` `                ``// move current to its right``                ``curr = curr.right;``            ``}` `            ``// restoring the tree back to original binary``            ``// search tree removing threaded links``            ``else``            ``{` `                ``succ.left = ``null``;` `                ``if` `(++count == k)``                    ``Klargest = curr;` `                ``// move current to its left child``                ``curr = curr.left;``            ``}``        ``}``    ``}``    ``return` `Klargest;``}` `// Driver code``public` `static` `void` `Main(String[] args)``{``    ``// Your C# Code``    ``/* Constructed binary tree is``        ``4``        ``/ \``    ``2 7``    ``/ \ / \``    ``1 3 6 10 */` `    ``Node root = newNode(4);``    ``root.left = newNode(2);``    ``root.right = newNode(7);``    ``root.left.left = newNode(1);``    ``root.left.right = newNode(3);``    ``root.right.left = newNode(6);``    ``root.right.right = newNode(10);` `    ``Console.Write(``"Finding K-th largest Node in BST : "` `+``                    ``KthLargestUsingMorrisTraversal(root, 2).data);``}``}` `// This code has been contributed by 29AjayKumar`

Javascript

 ``
Output:
`Finding K-th largest Node in BST : 7`

Time Complexity : O(n)
Auxiliary Space : O(1)

My Personal Notes arrow_drop_up