title: sparse matrix using linked list C++
body: <p>I'm coding the program that using linked list to store a sparse matrix. First I create a class "Node" contains the index of entry, value of entry and two pointers to next row and next column. Second I find on Google that I need to create the class Matrix like this code below but I don't understand the meaning of <code>Node **rowList</code> and <code>node** columnList</code>. Why they use a pointer to a pointer there and how could I implement a matrix from that? Thank you so much.</p>
  
  <pre><code>class Node
  {
  public:
      int iValue, jValue;
      float value;
      Node *rowPtr;
      Node *colPtr;
  };
  
  class Matrix
  {
      Node **rowList;     // rowList is the pointer to the array of rows
      Node **columnList;  // columnList is the pointer to the array of columns
      int rows, cols;     // number of rows and columns
  }
  </code></pre>