SDE - 1

1). Here, l used a stack for checking the validity of parentheses, and later remove the indexes of invalid parentheses from the string s. 

2). First, iterate the string s and mark the index of those characters which need to be removed to make it parentheses string using a special symbol '#'.

Here, a stack is used for finding the valid pair of parentheses, and while doing so also mark the indexes of invalid parentheses in s.

3). Finally, iterate s again and append non-marked symbol (#) to ans.

You have been given an integer ‘X’ and a non-decreasing sorted doubly linked list with distinct nodes.
Your task is to return the number of triplets in the list that sum up to the value ‘X’.
Input Format :
The first line contains a single integer ‘T’ denoting the number of test cases. The test cases follow.

The first line of each test case contains a single integer ‘X’.

The second line of the input contains the elements of the doubly linked list separated by a single space and terminated by -1. Hence, -1 would never be a list element.

You have been given an array/list 'PREORDER' representing the preorder traversal of a BST with 'N' nodes. All the elements in the given array have distinct values.
Your task is to construct a binary search tree that matches the given preorder traversal.
A binary search tree (BST) is a binary tree data structure that 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.

select *from employee 
group by salary 
order by salary desc limit 1,1;