Hash-Map

Construct a binary tree from preorder and inorder traversal sequences.

Create a deep copy of a linked list where each node has a random pointer

Find the minimum window substring that contains all characters of another string

Find the single number that appears once in an array where every other number appears twice.

Find the smallest substring that contains all characters of a given pattern

Sort characters in string by frequency using heap or bucket sort

Find the k most frequent elements in an array

Find two numbers in an array that add up to a target value.

Traverse a binary tree vertically and return nodes by columns