class Solution {
    public boolean isValidSudoku(char[][] board) {
        for(int i=0; i<board.length; i++)
        {
            for(int j=0; j<board[0].length; j++)
            {
                char ch = board[i][j];
                if(isValid(ch, i, j, board) == false)
                    return false;
            }
        }
        return true;
    }

    public boolean isValid(char ch, int row, int col, char[][] board)
    {
        if(board[row][col] == '.')
            return true;
        
        // traversing over 9*9 matrix
        for(int k=0; k<9; k++)
        {
            // checking in same row
            if(board[row][k] == ch && k!=col) // So, k != col ensures we ignore the cell we are currently trying to fill
                return false;

            // checking in same column
            if(board[k][col] == ch && k!=row) // So, k != row ensures we ignore the cell we are currently trying to fill
                return false;

            // starting row of the subgrid (3*3)
            int n = 3*(row/3) + k/3;
            // starting col of the subgrid (3*3)
            int m = 3*(col/3) + k%3;

            // we don’t want to compare the cell with itself.
            // If any other cell in the same 3×3 subgrid has ch, it’s not valid
            if( (n!=row || m!=col) && (board[n][m] == ch))
                return false;
        }
        return true;
    }
}