Finding all numbers that sum to a specified target number (integer partition)

Question

First i want to say i\'m still learning so my programming skills are not very good, and i\'m willing to accept any advice you have. Second i\'m still learning english so

Answer 1

Here's one approach that first builds the combinations into a tree structure, then arranges them into lists of ints:

internal class Combination
{
    internal int Num;
    internal IEnumerable<Combination> Combinations;
}

internal static IEnumerable<Combination> GetCombinationTrees(int num, int max)
{
    return Enumerable.Range(1, num)
                     .Where(n => n <= max)
                     .Select(n =>
                         new Combination
                         {
                             Num = n,
                             Combinations = GetCombinationTrees(num - n, n)
                         });
}

internal static IEnumerable<IEnumerable<int>> BuildCombinations(
                                               IEnumerable<Combination> combinations)
{
    if (combinations.Count() == 0)
    {
        return new[] { new int[0] };
    }
    else
    {
        return combinations.SelectMany(c =>
                              BuildCombinations(c.Combinations)
                                   .Select(l => new[] { c.Num }.Concat(l))
                            );
    }
}

public static IEnumerable<IEnumerable<int>> GetCombinations(int num)
{
    var combinationsList = GetCombinationTrees(num, num);

    return BuildCombinations(combinationsList);
}

public static void PrintCombinations(int num)
{
    var allCombinations = GetCombinations(num);
    foreach (var c in allCombinations)
    {
        Console.WriteLine(string.Join(", ", c));
    }
}

When run with the input value 6, this produces:

1, 1, 1, 1, 1, 1
2, 1, 1, 1, 1
2, 2, 1, 1
2, 2, 2
3, 1, 1, 1
3, 2, 1
3, 3
4, 1, 1
4, 2
5, 1
6