22 March 2015

Sudoku Solution Validator - Code Example

Five months ago, I was asked to solve a problem with a pair — the challenge was to write a Sudoku solution validator. We were expected to take a two-dimensional array that represented a Sudoku board and determine whether or not the board was ‘solved’ based on the rules of Sudoku.

Today, I attempted the same Sudoku challenge, but with two minor tweaks: I would be solo instead of with a pair, and I would deliver a solution in under one hour.

The account of this personal experiment will take the following form:

  1. statement and discussion of the Sudoku Solution Validator prompt
  2. explanation of my general approach
  3. overview of my solution logic
  4. solution code
  5. closing thoughts

Let’s get started!

Sudoku Solution Validator Prompt

Write a function validateSolution that accepts a 2-dimensional array representing a Sudoku board, and returns true if the board represents a valid solution, or false otherwise. The cells of the Sudoku board may contain the integers 1 through 9, also 0's, which will represent empty cells (source).

Example Expectations

General Approach

When I started the challenge, I set the following guidelines to focus my approach:

  1. modularize logic into small pieces about which it is easy to reason
  2. short-circuit unnecessary evaluation — check the easy fail-conditions first
  3. put off costly processing validations until the end
  4. use functional programming techniques within reason

Overview of Solution Logic

The logic of my solution is stupid-simple.

Check that each row, column, and region sums to 45 and contains one and only one of each possible integer between 1 and 9.

To optimize performance, each step can short-circuit the whole process if any of the checks return false.

In order to ‘check’ each entity (row, column, or region), I followed this process:

  1. convert row/column/region into array of 9 numbers
  2. check that sum of array members is 45
  3. check that the numbers 1 through 9 each appear once (and only once) in the array

If all of the checks succeed, the solution is valid and we return true.

Pretty simple, eh? Let’s see the code!


I had a lot of fun with this challenge, though I personally hate Sudoku.