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Project Euler 14

Project Euler

Problem14を解き直しました.

  • Longest Collatz sequence

The following iterative sequence is defined for the set of positive integers:

n → n/2 (n is even)
n → 3n + 1 (n is odd)

Using the rule above and starting with 13, we generate the following sequence:

13 → 40 → 20 → 10 → 5 → 16 → 8 → 4 → 2 → 1
It can be seen that this sequence (starting at 13 and finishing at 1) contains 10 terms. Although it has not been proved yet (Collatz Problem), it is thought that all starting numbers finish at 1.

Which starting number, under one million, produces the longest chain?

NOTE: Once the chain starts the terms are allowed to go above one million.

https://projecteuler.net/problem=14

問題は「100万未満の自然数の中で,最長のコラッツ数列を生成する自然数を求めよ」.

def getCollatzLen(num)
  len = 1
  while num != 1
    if num.odd?
      num = 3 * num + 1
    else
      num /= 2
    end
    len += 1
  end
  len
end

max = 0
num = 0
for i in 1..999_999 do
  if max < getCollatzLen(i)
    max = getCollatzLen(i)
    num = i
  end
end

p num