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This week, we will solve the most popular number puzzle on the planet. Back in the day, many people would buy a newspaper just to solve this puzzle. Are you ready?

Today we will cover:

• Valid Sudoku

• Optimistic vs Pessimistic Locking

Read time: under 4 minutes

CODING CHALLENGE

Valid Sudoku

Determine if a 9 x 9 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.

2. Each column must contain the digits 1-9 without repetition.

3. Each of the nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without repetition.

Note:

A Sudoku board (partially filled) could be valid but is not necessarily solvable.

Only the filled cells need to be validated according to the mentioned rules.

Example 1:

Input: board = 
[["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: board = 
[["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.

Solve the problem here before reading the solution.

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SOLUTION

To solve this problem, we'll validate the Sudoku board by checking three conditions:

1. No repetition in rows

2. No repetition in columns

3. No repetition in 3x3 sub-boxes

We'll use sets to efficiently track unique digits in each row, column, and sub-box. By checking each condition during a pass through the board, we can determine its validity.

Time complexity is O(1) since the board size is fixed at 9x9. Space complexity is also O(1) as we're using a constant amount of extra space.

SYSTEM DESIGN

Optimistic vs Pessimistic Locking

When multiple users try to update the same data in a database simultaneously, it can lead to data inconsistency. For example, if two bank tellers try to update a customer's balance at the same time, one update might override the other. Database locking helps prevent such conflicts.

There are two main approaches to handle concurrent updates: Optimistic Locking and Pessimistic Locking.

Optimistic Locking assumes conflicts are rare. It lets users make changes freely but checks for conflicts before saving. Each record has a version number that increases with each update. When saving changes, the system verifies if the last version number matches what it was when the user started editing. If someone else made changes in between, the version numbers won't match, and the save fails.

This approach works well for systems where conflicts are uncommon, like social media posts where users typically edit their own content. It's also more efficient since it doesn't block other users from reading or editing.

Pessimistic Locking takes a more cautious approach. When a user starts editing a record, it gets locked immediately. Other users must wait until the first user finishes.

This approach is better for systems where conflicts are likely and data consistency is crucial, like banking transactions. But it can create performance issues if locks are held for too long, as other users are forced to wait.

Here's how they compare:

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BONUS

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Sahil + Sarra