euler14-memo.rb

MAXSIZE = 1000000
MEMO = Array.new(MAXSIZE)

def collatz_size(i)
  size = 0
  n = i
  while n != 1 do
    if n <= MAXSIZE && MEMO[n]
      size += MEMO[n]
      break
    else
      n = n.even? ? n/2 : 3*n + 1
      size += 1
    end
  end
  MEMO[i] = size  #メモ化
end

max = (1...MAXSIZE).each.map do |i|
  { input: i, count: collatz_size(i) }
end.max {|i, j| i[:count] <=> j[:count] }
puts max[:input]

# 837799
# 3.2s CPU: Core i5 1.6GHz, MEM: 4G