Nine notes

Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.

Given a binary tree, determine if it is a valid binary search tree (BST).

Given a binary tree, return all root-to-leaf paths.

Given a stream of integers and a window size, calculate the moving average of all integers in the sliding window.

Median is the middle value in an ordered integer list. If the size of the list is even, there is no middle value. So the median is the mean of the two middle value.

We are playing the Guess Game. The game is as follows:

Given an encoded string, return it’s decoded string.

There is a fence with n posts, each post can be painted with one of the k colors.

The cost of painting each house with a certain color is represented by a n x k cost matrix. For example, costs[0][0] is the cost of painting house 0 with color 0; costs[1][2] is the cost of painting house 1 with color 2, and so on… Find the minimum cost to paint all houses. Could you solve it in O(nk) runtime?

Given a sorted integer array where the range of elements are [lower, upper] inclusive, return its missing ranges.

Given an array nums, there is a sliding window of size k which is moving from the very left of the array to the very right. You can only see the k numbers in the window. Each time the sliding window moves right by one position.

Given an array of strings, group anagrams together.

Given a non-empty array of integers, return the k most frequent elements.

Given two strings s and t which consist of only lowercase letters.