## Invert Binary Tree

Invert a binary tree so that the left and right children of each node swap.

## Valid Palindrome

A palindrome is a word, phrase, number, or other sequence of characters which reads the same backward or forward. Given a string, determine if it is a palindrome, considering only alphanumeric characters and ignoring cases.

## Lowest Common Ancestor

Given the root and two nodes in a binary tree. Find the lowest common ancestor(LCA) of the two nodes.

## Valid Sudoku

Given a Sudoku board, write an algorithm to determine if the board is valid.

## Insert Node in a Binary Search Tree

Given a binary search tree and a new tree node, insert the node into the tree. You should keep the tree still be a valid binary search tree.

## Ugly Number II

Ugly number is a number that only have factors `2`

, `3`

and `5`

. Design an algorithm to find the nth ugly number. The first 10 ugly numbers are `1, 2, 3, 4, 5, 6, 8, 9, 10, 12, ...`

## Binary Search Tree Iterator

Design an iterator over a binary search tree with the following rules:

## Cosine Similarity

Cosine similarity is a measure of similarity between two vectors of an inner product space that measures the cosine of the angle between them. The cosine of 0° is 1, and it is less than 1 for any other angle. It is defined as follows:

## Longest Common Prefix

Given k strings, find the longest common prefix (LCP).

## First Bad Version

The code base version is an integer start from 1 to n. One day, someone committed a bad version in the code case, so it caused this version and the following versions are all failed in the unit tests. Find the first bad version.