How to Check if given Sudoku is Valid or not - The Coding Shala

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How to Check if given Sudoku is Valid or not

In this post, you will learn how to check if given Sudoku is valid or not. We will write a Java program to check if the given sudoku board is valid.

Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules:
  1. Each row must contain the digits 1-9 without repetition.
  2. Each column must contain the digits 1-9 without repetition.
  3. Each of the 9 3x3 sub-boxes of the grid must contain the digits 1-9 without repetition.
The Sudoku board could be partially filled, where empty cells are filled with the character '.'.

Example:
For
grid = [['.', '.', '.', '1', '4', '.', '.', '2', '.'],

           ['.', '.', '6', '.', '.', '.', '.', '.', '.'],

           ['.', '.', '.', '.', '.', '.', '.', '.', '.'],

           ['.', '.', '1', '.', '.', '.', '.', '.', '.'],

           ['.', '6', '7', '.', '.', '.', '.', '.', '9'],
           ['.', '.', '.', '.', '.', '.', '8', '1', '.'],
           ['.', '3', '.', '.', '.', '.', '.', '.', '6'],
           ['.', '.', '.', '.', '.', '7', '.', '.', '.'],
           ['.', '.', '.', '5', '.', '.', '.', '7', '.']]
Output should be true.
Check if sudoku is valid or not - The Coding Shala

Java Program for Valid Sudoku

Approach 1:
In Sudoku we need to verify three things. Each row and column must contain the digits 1-9 without repetition and third is Each of the 9, 3*3 boxes of the grid must contain the digits 1-9 without repetition. We can check these things using 3 HashSets or we can take three arrays.
Java Program: 

boolean sudoku2(char[][] grid) {
    for(int i=0;i<9;i++){
        HashSet<Character> row = new HashSet<>();
        HashSet<Character> col = new HashSet<>();
        HashSet<Character> box = new HashSet<>();
        for(int j=0;j<9;j++){
            if(grid[i][j]!='.'){
                if(row.contains(grid[i][j])) return false;
                else row.add(grid[i][j]);
            }

            if(grid[j][i]!='.'){
                if(col.contains(grid[j][i])) return false;
                else col.add(grid[j][i]);
            }

            int row_index = 3*(i/3);
            int col_index = 3*(i%3);
            if(grid[row_index + (j/3)][col_index + (j%3)]!='.'){
                if(box.contains(grid[row_index + (j/3)][col_index + (j%3)]))
                    return false;
                else box.add(grid[row_index + (j/3)][col_index + (j%3)]);
            }
        }
    }
    return true;
}


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