Problem Description

You are given a 9 x 9 Sudoku board board. A Sudoku board is valid if the following rules are followed:

1. Each row must contain the digits 1-9 without duplicates.
2. Each column must contain the digits 1-9 without duplicates.
3. Each of the nine 3 x 3 sub-boxes of the grid must contain the digits 1-9 without duplicates.

Return true if the Sudoku board is valid, otherwise return false.

Note: A board does not need to be full or be solvable to be valid.

Example 1:

Input: board = 
[["1","2",".",".","3",".",".",".","."],
 ["4",".",".","5",".",".",".",".","."],
 [".","9","8",".",".",".",".",".","3"],
 ["5",".",".",".","6",".",".",".","4"],
 [".",".",".","8",".","3",".",".","5"],
 ["7",".",".",".","2",".",".",".","6"],
 [".",".",".",".",".",".","2",".","."],
 [".",".",".","4","1","9",".",".","8"],
 [".",".",".",".","8",".",".","7","9"]]

Output: true

Example 2:

Input: board = 
[["1","2",".",".","3",".",".",".","."],
 ["4",".",".","5",".",".",".",".","."],
 [".","9","1",".",".",".",".",".","3"],
 ["5",".",".",".","6",".",".",".","4"],
 [".",".",".","8",".","3",".",".","5"],
 ["7",".",".",".","2",".",".",".","6"],
 [".",".",".",".",".",".","2",".","."],
 [".",".",".","4","1","9",".",".","8"],
 [".",".",".",".","8",".",".","7","9"]]

Output: false
Explanation: There are two 1's in the top-left 3x3 sub-box.

Constraints:

• board.length == 9
• board[i].length == 9
• board[i][j] is a digit 1-9 or '.'.

Approach

The approach involves validating the Sudoku board by checking three conditions:

1. Row Validation: Ensure no digit 1-9 repeats in any row.
2. Column Validation: Ensure no digit 1-9 repeats in any column.
3. Sub-box Validation: Ensure no digit 1-9 repeats in any of the nine 3x3 sub-grids.

We can use HashSet to keep track of seen numbers for each row, column, and sub-box. If a duplicate is found, the board is invalid.

Solution

import java.util.ArrayList;
import java.util.HashSet;

class Solution {
    public boolean isValidSudoku(char[][] board) {
        int i=0;
        while(i<9){
            ArrayList<Character> arr = new ArrayList<>();
            HashSet<Character> ht = new HashSet<>();
            int j=0;
            while(j<9){
                if(board[i][j]!='.'){
                    arr.add(board[i][j]);
                    ht.add(board[i][j]);
                }
                j++;
            }
            if(arr.size()!=ht.size()){
                return false;
            }
            arr = new ArrayList<>();
            ht = new HashSet<>();
            int k=0;
            while(k<9){
                if(board[k][i]!='.'){
                    arr.add(board[k][i]);
                    ht.add(board[k][i]);
                }
                k++;
            }
            if(arr.size()!=ht.size()){
                return false;
            }
            i++;
        }
        for(i=0;i<9;i=i+3){
            for(int j=0;j<9;j=j+3){
                HashSet<Character> ht = new HashSet<>();
                for(int k=i;k<i+3;k++){
                    for(int m=j;m<j+3;m++){
                        if(board[k][m]!='.'){
                            if(!ht.add(board[k][m])){
                                return false;
                            }

                        }
                    }
                }
            }
        }
        return true;
    }
}