Last Updated: 19 Nov, 2020

BST to greater tree

Easy
Asked in company
Arcesium

Problem statement

Given a binary search tree of integers with N number of nodes. Your task is to convert it to a Greater Tree.

A Greater Tree is formed when data of every node of the original BST is changed to the original node’s data plus the sum of all node’s data greater than or equal to it.

A binary search tree (BST) is a binary tree data structure which has the following properties.

• The left subtree of a node contains only nodes with data less than the node’s data.
• The right subtree of a node contains only nodes with data greater than the node’s data.
• Both the left and right subtrees must also be binary search trees.
For example:

Input:

Example

Output:

Because only 5 is greater than 4 in the above BST, node 4 will be updated to 9 (4 + 5 = 9).
Nodes 3, 4, and 5 are greater than 2, hence node 2 will be modified to 14 (2 + 3 + 4 + 5 = 14). 
Node with data 5 will remain the same because there is no node in the BST with data greater than 5.
Nodes 2,3, 4, and 5 are greater than 1, hence node 1 will be modified to 15 (1 + 2 + 3 + 4 + 5 = 15).   
Nodes 4 and 5 are greater than 3, hence node 3 will be modified to 12 (3 + 4 + 5 = 12). 
Input Format:
The first line contains an integer 'T' which denotes the number of test cases or queries to be run. Then the test cases follow.

The only line of each test case contains the elements of the tree in the level order form separated by a single space.
If any node does not have a left or right child, take -1 in its place. Refer to the example below.

Example:

Elements are in the level order form. The input consists of values of nodes separated by a single space in a single line. In case a node is null, we take -1 in its place.

For example, the input for the tree depicted in the below image would be :

Example

1
2 3
4 -1 5 6
-1 7 -1 -1 -1 -1
-1 -1

Explanation :
Level 1 :
The root node of the tree is 1

Level 2 :
Left child of 1 = 2
Right child of 1 = 3

Level 3 :
Left child of 2 = 4
Right child of 2 = null (-1)
Left child of 3 = 5
Right child of 3 = 6

Level 4 :
Left child of 4 = null (-1)
Right child of 4 = 7
Left child of 5 = null (-1)
Right child of 5 = null (-1)
Left child of 6 = null (-1)
Right child of 6 = null (-1)

Level 5 :
Left child of 7 = null (-1)
Right child of 7 = null (-1)

The first not-null node (of the previous level) is treated as the parent of the first two nodes of the current level. The second not-null node (of the previous level) is treated as the parent node for the next two nodes of the current level and so on.

The input ends when all nodes at the last level are null (-1).

Note :
The above format was just to provide clarity on how the input is formed for a given tree. 

The sequence will be put together in a single line separated by a single space. Hence, for the above-depicted tree, the input will be given as:

1 2 3 4 -1 5 6 -1 7 -1 -1 -1 -1 -1 -1
Output Format :
For each test case, print the level order traversal of the Greater Tree with node values separated by a single space. Print -1 for NULL nodes. 

Output for every test case will be denoted in a separate line.

Note: 
You are not required to print anything explicitly. It has already been taken care of. Just implement the function.
Constraints :
1 <= T <= 100
1 <= N <= 5000
0 <= data <= 10^6
where N is the number of nodes in the tree, T represents the number of test cases and data denotes data contained in the node of the binary tree.


Time Limit: 1 sec

Approaches

01 Approach

  • This is a basic brute force approach, in which for every single node, we will traverse the whole tree to find all the elements which are greater or equal to the current node.
  • First, we will store all the nodes in an array or a similar data structure.
  • Further, we will traverse the tree again and for each node of the tree, we will loop over the array and find the sum of all the elements which are greater than or equal to the current node.
  • We will modify the current node with the new value obtained, and hence the binary search tree gets converted to a greater tree.

02 Approach

  • We will use reverse inorder traversal so that we can traverse the tree in descending order.
  • We can see a pattern while solving this problem. The value of the 2nd largest node would be the sum of the current node and the largest node.
  • Similarly, the value of the Nth largest node will be the sum of the current node and the sum of (N-1)th largest node after modification.
  • So we can traverse the tree in descending order and simultaneously update the sum value at every step while adding the value to the current node.

03 Approach

  • Using stack, we can perform the reverse inorder traversal on the BST iteratively.
  • Firstly, we will initialize  ‘currNode’ with the root node, and maintain a variable ‘sum’ with an initial value of 0.
  • We will loop until the stack becomes empty.
    • We iteratively traverse to the rightmost node of the tree and while traversing, we will push ‘currNode’ onto the stack.
    • Now, we pop an element from the stack, add it to the sum and update the value of that node to the ‘sum’.
    • Then, we iteratively traverse through the left subtree, which means we will update ‘currNode’ to the left child of ‘currNode’.