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Grokking the Coding Interview Patterns in Python

Sudoku Solver

Explore how to solve a 9x9 Sudoku puzzle by applying the backtracking algorithm. Understand how to validate rows, columns, and 3x3 sub-boxes to complete the board correctly. This lesson helps you develop problem-solving skills crucial for coding interviews, especially those involving constraint satisfaction and recursive solutions.

We'll cover the following...

Statement

Given a 9 x 9 sudoku board, solve the puzzle by completing the empty cells. The sudoku board is only considered valid if the following rules are satisfied:

  • Each row must contain digits between 1 and 9, and there should be no repetition of digits within a row.

  • Each column must contain digits between 1 and 9, and there should be no repetition of digits within a column.

  • The board consists of 9 non-overlapping sub-boxes, each containing 3 rows and 3 columns. Each of these 3 x 3 sub-boxes must contain digits between 1 and 9, and there should be no repetition of digits within a sub-box.

Constraints:

  • board.length =9= 9

  • board[i].length =9= 9

  • board[i][j] is a digit or ..

  • The input board is guaranteed to have one solution.

Example

Understand the problem

Let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

Sudoku Solver

1.

Is the 9 x 9 sudoku board given below valid?

1 2 3   4 5 6   7 8 9
7 8 9   1 2 3   4 5 6
4 5 6   7 8 9   1 2 3

3 1 2   8 4 5   9 6 7
6 9 7   3 1 2   8 4 5
8 4 5   6 9 7   3 1 2

2 3 1   5 7 4   6 9 8
9 6 8   2 3 1   5 7 4
5 7 4   9 6 8   2 3 1
A.

Yes

B.

No

1 / 2

Figure it out!

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Note: As an additional challenge, we have intentionally hidden the solution to this puzzle.

Sequence - Vertical
Drag and drop the cards to rearrange them in the correct sequence.
1
2
3
4
5
6

Try it yourself

Implement your solution in the following coding playground.

We have left the solution to this challenge as an exercise for you. An optimal solution to this problem runs in O(9(n*m)) time and takes O(n*m) space. You may try to translate the logic of the solved puzzle into a coded solution.

Python
usercode > main.py
def solve_sudoku(board):

    # Replace this placeholder return statement with your code

    

    return []
Sudoku Solver