BST to sorted DLL

Moderate

0/80

Average time to solve is 50m

33 upvotes

Asked in companies

Problem statement

You are provided with a Binary Search Tree (BST), all you have to do is to convert it into the sorted doubly linked list (DLL).

For Example:

Consider the above BST, it will be converted into the below sorted DLL.

Here, 20 is the head node and 80 is the tail node.

Detailed explanation ( Input/output format, Notes, Images )

Input Format:

The first line contains a single integer ‘T’ representing the number of test cases. 

The first line of each test case contains elements of the tree in the level order form. The line consists of values of nodes separated by a single space. 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 :

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 according to the Binary Search tree. 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 elements of the sorted doubly linked list in the space-separated form.

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

Note:

You don’t need to print anything; It has already been taken care of. Just implement the given function.

Follow up:

Try to do the problem in order O(N) time complexity or less for getting the solution accepted.

Constraints:

1 <= T <= 10
0 <= N <= 10^4
-10^5 <= DATA <= 10^5

Time Limit: 1sec

Sample Input 1:

2
50 30 70 20 40 60 80 -1 -1 -1 -1 -1 -1 -1 -1
-1

Sample Output 1:

20 30 40 50 60 70 80 -1
-1

Explanation Of Sample Input 1:

For the first test case, the explanation is given in the description. -1 represents the end of DLL.

In the second test case, there is no node in BST and so, there is also no node in DLL.

Sample Input 2:

2
0 -2 -1 -3 -1 -1 -1
1 -1 2 -1 3 -1 -1

Sample Output 2:

-3 -2 0 -1
1 2 3 -1

Explanation Of Sample Input 2:

In the first test case, the sorted DLL formed is [-3, -2, 0].

In the second test case, the sorted DLL formed is [1, 2, 3].

Console