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Two dimensional Matrix will consume lot of memory...so we can use sparse matrix, where we can store non-null nodes. We can store the sparse matrix using linked lists inorder to utilize memory efficiently....

Hashtable of another Hashtable

First table will store row ids against another Hashtable,which maps the column of that row to values of column.In a very decent way (some kind of hGrid)

I would use normal 2D matrix, with virtually allocated memory, commiting only the used cells. See VirtualAlloc() function in win32 for details. In this case this is very easy to use and memory consumption is just as needed. Using any of the suggested above methods will require keeping track of the unused and used cells - this code is going to be unmanageable...