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Decode the Coding Interview in Ruby: Real-World Examples

Sudoku Solver

Explore techniques to solve Sudoku puzzles by filling empty cells in a 9x9 grid. Understand how to use backtracking and constraint programming to ensure each digit appears only once per row, column, and sub-box. This lesson helps you implement an algorithm that handles trial, error, and correction to complete the puzzle efficiently.

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Description

Write a program to solve Sudoku by filling the empty cells. We will be given a 2D array representing a Sudoku puzzle. For a correct solution each of the digits 1-9 must only occur once in each:

  • Row
  • Column
  • Nine 3x3 sub-boxes of the grid

Let us take a look at an example:

To solve the puzzle, we will only fill the empty cells. Empty cells are represented by a ‘.’. The array will be passed by reference, so we will not need to return it after solving the puzzle.

Coding exercise

Ruby
def solve_sudoku(board)
  # Write your code below
end
Sudoku solver

Solution

While solving Sudoku, we will have to take care of two things. First, we place a number at an empty place. The number must not be present in that row, column, or sub-box. So, we will use constrained programming to keep track of which number we can place in a box and which not. Second, let us assume that we have filled a few empty places but, the choice of numbers was not optimal, ...