trjhtr

can you give me your impression of my implementation of Pascal triangle problem?
I'm especially interested in space complexity evaluation
thank you all!

public List<List<int>> generate(int A)

 List<List<int>> result = new List<List<int>>();

 int currenctRowLength = 1;
 List<int> previousRow = new List<int>();
 for (int i = 0; i < A; i++)
 
 List<int> row = new List<int>();
 row.Add(1);
 currenctRowLength = i + 1;
 int j = 1;
 while (j < currenctRowLength - 1)
 
 row.Add(previousRow[j] + previousRow[j - 1]);
 j++;
 

 if (i > 0)
 
 row.Add(1);
 

 previousRow=row;
 result.Add(row);
 
 return result;

As I think greybeard was trying to say, unless you are know that memory consumption/allocation costs are a serious concern, then this code is fine from a memory point of view. The space-complexity is the same as the output (quadratic in A), so you can't do better than that.

There a couple of small things to be said:

• as GreyBeard says, A is completely meaningless, and should really have a lowerCamelCase name as a parameter (e.g. dimension, size?); generate ought to be GeneratePascalTriangle or something meaningful (public -> UpperCamelCase) (msdn reference)




• currenctRowLength should be currentRowLength, and should only be defined inside the for loop (i.e. where it is assigned); I like that this is its own variable.




• you are using a while loop over j, when a for could be more readable (simply because people are used to reading for-loops), and doesn't leak j into the outer scope.




• List<List<int>> is a pretty horrid type to be returning: much better to have IReadOnlyList<IReadOnlyList<T>> if the return value is meant to be immutable (which is assignable from List<List<T>> or T).




• I'd be inclined to initialise previousRow to null, rather than an empty list: it isn't read until i == 2; assigning it to a 'meaningful' value is defensive and only can work to obscure bugs in the rest of the code.

Mindless Performance Musing

If memory usage/allocations/GC hammering is a real concern, then you can improve matters by not using resizing lists. You havn't given us any idea of what the code is actually meant to do, so it's a bit hard to suggest an alternative, but you could either use arrays (which obvious are non-dynamic) or you can preserve the signature by creating lists with a given initial capacity with the .ctor(int) overload. Note that this will give a performance benefit, but change the time-complexity (which is also quadratic in A).

Code

Putting all of that together (and using arrays instead of lists, because mindless performance is fun, and I don't think it harms the readability here much):

/// <summary>
/// Generates a pascal triangle with the given number of rows
/// </summary>
public static IReadOnlyList<IReadOnlyList<int>> GeneratePascalTriangle(int numberOfRows)

 int result = new int[numberOfRows];

 int previousRow = null;
 for (int i = 0; i < numberOfRows; i++)
 
 int currentRowLength = i + 1;

 int row = new int[currentRowLength];
 result[i] = row;

 // start and end
 row[0] = 1;
 row[currentRowLength - 1] = 1;

 // middle
 for (int j = 1; j < currentRowLength - 1; j++)
 
 row[j] = previousRow[j] + previousRow[j - 1];
 

 previousRow = row;
 

 return result;