You are given the root of a binary tree. The task is to return the preorder traversal of the tree. Preorder traversal means visiting the root node first, followed by the left subtree, and then the right subtree.
You are given the root of a binary tree. Imagine yourself standing on the right side of the tree, and return the values of the nodes you can see when viewed from the right, ordered from top to bottom.
You are given a 2D grid representing a map, where ‘1’ represents land and ‘0’ represents water. Your task is to count how many islands are formed by connecting adjacent lands horizontally or vertically. An island is a collection of ‘1’s connected either horizontally or vertically.
You are given a set of courses and a list of prerequisites. Each prerequisite is a pair of courses where the second course must be taken before the first one. Determine if it is possible to complete all courses based on these prerequisites. If there are cycles in the dependencies, it would be impossible to finish all courses.
You are tasked with creating a data structure that allows adding words and checking if a word matches any previously added word, with support for the wildcard ‘.’ character.
