Introduction

In this blog post, we will explore how to solve the LeetCode problem "36" using AI. LeetCode is a popular platform for preparing for coding interviews, and with the help of AI tools like Chatmagic, we can generate solutions quickly and efficiently - helping you pass the interviews and get the job offer without having to study for months.

Problem Statement

Determine if a 9 x 9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules: Each row must contain the digits 1-9 without repetition. Each column must contain the digits 1-9 without repetition. 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. Constraints: board.length == 9 board[i].length == 9 board[i][j] is a digit 1-9 or '.'.

Explanation

Here's a breakdown of the solution:

• Use Sets for Efficient Duplicate Detection: The core idea is to iterate through rows, columns, and 3x3 sub-boxes and use sets to keep track of the numbers encountered. If a number is already present in the set, it indicates a duplicate, making the Sudoku board invalid.

• Iterate and Validate: The solution iterates through the board, validating each row, column, and 3x3 sub-box. The 3x3 sub-box validation is done efficiently using integer division to determine the starting row and column index of each sub-box.

• Time & Space Complexity: O(1) time complexity (since the board size is fixed at 9x9) and O(1) space complexity (we use a constant number of sets, regardless of the input).

Code

    def isValidSudoku(board):
    """
    Determines if a 9 x 9 Sudoku board is valid.

    Args:
        board: A list of lists of strings representing the Sudoku board.

    Returns:
        True if the Sudoku board is valid, False otherwise.
    """

    def is_valid_subset(subset):
        seen = set()
        for val in subset:
            if val != '.':
                if val in seen:
                    return False
                seen.add(val)
        return True

    # Check rows
    for row in board:
        if not is_valid_subset(row):
            return False

    # Check columns
    for col in range(9):
        column = [board[row][col] for row in range(9)]
        if not is_valid_subset(column):
            return False

    # Check 3x3 sub-boxes
    for i in range(3):
        for j in range(3):
            sub_box = [board[row][col] for row in range(i*3, (i+1)*3) for col in range(j*3, (j+1)*3)]
            if not is_valid_subset(sub_box):
                return False

    return True