Arrays & Hashing - Valid Sudoku

Valid Sudoku

Problem Information

Leetcode problem: https://leetcode.com/problems/valid-sudoku/description/
Neetcode problem: https://neetcode.io/problems/valid-sudoku
Difficulty: Medium

Solution

The problem asks you to iterate the board the smartest possible way that you can. To be able to check it in one pass, you need to calculate a square or box index.

class Solution {
    public boolean isValidSudoku(char[][] board) {
        int boardSize = board.length;
        var columns = new ArrayList<HashSet<Character>>(boardSize);
        var rows = new ArrayList<HashSet<Character>>(boardSize);
        var squares = new HashMap<String, HashSet<Character>>();

        for (int i = 0; i < boardSize; i++) {
            columns.add(i, new HashSet<Character>());
            rows.add(i, new HashSet<Character>());
        }

        for (int r = 0; r < board.length; r++) {
            for (int c = 0; c < board.length; c++) {
                var num = board[r][c];
                if (num != '.') {
                    var columnSet = columns.get(c);
                    var rowSet = rows.get(r);

                    var squareKey = r/3 + "," + c/3;
                    squares.putIfAbsent(squareKey, new HashSet<Character>());
                    var squareSet = squares.get(squareKey);

                    if (columnSet.contains(num))
                        return false;
                    else
                        columnSet.add(num);

                    if (rowSet.contains(num))
                        return false;
                    else
                        rowSet.add(num);

                    if (squareSet.contains(num))
                        return false;
                    else
                        squareSet.add(num);

                }
            }
        }

        return true;
    }
}