38. Valid Sudoku

Topic :

hashing

Difficulty :

medium

Problem Link :

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 '.'.

solution

TIME COMPLEXITY : O(9^2)

import java.io.*;
import java.util.*;
class ValidSudoku {
    static boolean isValidSudoku(char[][] board) {
        HashSet < String > set = new HashSet < > ();
        for (int i = 0; i < 9; i++) {
            for (int j = 0; j < 9; j++) {
                char number = board[i][j];

                if (number != '.') {
                    if (!set.add(number + " present in row " + i) ||
                    !set.add(number + " present in col " + j) ||
                    !set.add(number + " present in box " + (i / 3) + '/' + (j / 3)))

                        return false;
                }
            }
        }

        return true;
    }

    public static void main(String args[]){
        char 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'}};

        System.out.println(isValidSudoku(board));
    }
}