Find two numbers that add up to a target using hash map.

Check if parentheses string is valid using stack approach.

Reverse a singly linked list using iterative approach.

Merge two strings by alternating their characters.

Check if an integer is a palindrome without converting to string.

Merge two sorted linked lists using dummy node approach.

Traverse binary tree inorder using recursive and iterative approaches.

Add two numbers represented as linked lists with carry handling.

Find the longest substring without repeating characters using sliding window technique.