Project Euler Problem #52

Read the details of the problem here

Summary

Find the smallest positive integer, x, such that 2x, 3x, 4x, 5x, and 6x, contain the same digits in some order.

Solution

I used Groovy listops to drive this in, in essence, a single line within a loop. It was quick to write and “obviously correct” but very slow – taking 2.82 seconds. Using as Set is faster than using groupBy() to eliminate duplicates.

def ( i, answer ) = [ 1, 0 ]

while (answer == 0) {

  answer =
      ( ((1..6).collect { (i * it).toString().toList().sort() } as Set)
               .size() == 1 ) ? i : 0

  i++
}

Removing the reliance on the list operators, as per the solution below, brings the runtime down to 0.78 seconds. The code isn’t really any more complex.

def ( i, answer ) = [ 1, 0 ]

while (answer == 0) {

  def s = (i.toString() as List).sort()

  for (j in 2..6) {
    if (s != (i * j).toString().toList().sort()) break
    if (j == 6) answer = i
  }

  i++
}

So that’s the two-for-one deal on this solution.

Conclusion

Groovy – You can take declarative logic, or some moderate performance. Take your pick.

Note: Putting this performance into context though, a port of the solutions to Ruby 1.8.7 has them running in 20.2 seconds and 4.55 seconds respectively. Running these scripts in JRuby 1.5.0 gives run times of 5.57 seconds and 1.60 seconds respectively. So both Groovy and the JVM scores very well in this regard.