Hash Table Practice

In this exercise you will complete two coding problems using Hash Tables.

Learning Goals

By the end of this exercise you should be able to:

Use a hash table to solve a coding problem
Identify how a hash table can produce a more attractive runtime over alternative solutions

Grouped Anagrams

Given an array of strings, group anagrams together.

Example

Input: ["eat", "tea", "tan", "ate", "nat", "bat"],
Output:
[
  ["ate","eat","tea"],
  ["nat","tan"],
  ["bat"]
]

Note:

All inputs will be in lowercase.
The order of your output does not matter

Most Frequent K Elements

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

Example 1

Input: nums = [1,1,1,2,2,3], k = 2
Output: [1,2]

Example 2

Input: nums = [1], k = 1 Output: [1]


## Note

You may assume k is always valid, 1 ≤ k ≤ number of unique elements.

You should be able to equal or beat O(n log n), where n is the array's size.


## Optional: Valid Sudoku

Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:

1. Each row must contain the digits 1-9 without repetition.
1. Each column must contain the digits 1-9 without repetition.
1. Each of the 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.

![sudoku image](images/250px-Sudoku-by-L2G-20050714.svg.png)

Above is a valid Sudoku grid.

The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

**Example 1:**

Input: [ ["5","3",".",".","7",".",".",".","."], ["6",".",".","1","9","5",".",".","."], [".","9","8",".",".",".",".","6","."], ["8",".",".",".","6",".",".",".","3"], ["4",".",".","8",".","3",".",".","1"], ["7",".",".",".","2",".",".",".","6"], [".","6",".",".",".",".","2","8","."], [".",".",".","4","1","9",".",".","5"], [".",".",".",".","8",".",".","7","9"] ] Output: true



**Example 2:**

Input: [ ["8","3",".",".","7",".",".",".","."], ["6",".",".","1","9","5",".",".","."], [".","9","8",".",".",".",".","6","."], ["8",".",".",".","6",".",".",".","3"], ["4",".",".","8",".","3",".",".","1"], ["7",".",".",".","2",".",".",".","6"], [".","6",".",".",".",".","2","8","."], [".",".",".","4","1","9",".",".","5"], [".",".",".",".","8",".",".","7","9"] ] Output: false Explanation: Same as Example 1, except with the 5 in the top left corner being modified to 8. Since there are two 8's in the top left 3x3 sub-box, it is invalid.