Fast solution to Subset sum

Question

Consider this way of solving the Subset sum problem:

def subset_summing_to_zero (activities):
  subsets = {0: []}
  for (activity, cost) in activities.iterit         


        

Answer 1

Thought I'd share my Scala solution for the discussed pseudo-polytime algorithm described in wikipedia. It's a slightly modified version: it figures out how many unique subsets there are. This is very much related to a HackerRank problem described at https://www.hackerrank.com/challenges/functional-programming-the-sums-of-powers. Coding style might not be excellent, I'm still learning Scala :) Maybe this is still helpful for someone.

object Solution extends App {
    var input = "1000\n2"

    System.setIn(new ByteArrayInputStream(input.getBytes()))        

    println(calculateNumberOfWays(readInt, readInt))

    def calculateNumberOfWays(X: Int, N: Int) = {
            val maxValue = Math.pow(X, 1.0/N).toInt

            val listOfValues = (1 until maxValue + 1).toList

            val listOfPowers = listOfValues.map(value => Math.pow(value, N).toInt)

            val lists = (0 until maxValue).toList.foldLeft(List(List(0)): List[List[Int]]) ((newList, i) => 
                    newList :+ (newList.last union (newList.last.map(y => y + listOfPowers.apply(i)).filter(z => z <= X)))
            )

            lists.last.count(_ == X)        

    }
}