Euler3 and Euler4

gistfile1.rb

def euler3
  num=600851475143
  n=num
  if n % 2 ==0
    lastFactor = 2
    n = n/2
    while n%2==0
      n = n/2
    end
  else
    lastFactor = 1
  end
  currentFactor = 3
  maxFactor = Math.sqrt(num)
  while n>1 && currentFactor<=maxFactor
    if n % currentFactor ==0
      n = n/currentFactor
      lastFactor = currentFactor
      while n%currentFactor==0
        n=n/currentFactor
      end
      maxFactor = Math.sqrt(n)
    end
    currentFactor = currentFactor+2
  end
  final = (n==1) ? lastFactor : n
end

def euler4
  largestPalindrome = 0
  a = 100
  while a <= 999
    b = a
    while b <=999
      largestPalindrome = a*b if palindrome?(a*b) && a*b>largestPalindrome
      b = b+1
    end
    a = a+1
  end
 largestPalindrome
end

def palindrome?(num)
  (num).to_s.reverse.to_i == (num)
end


def sieve(num)
  arr = Array.new(num,true)
  arr[0]=false
  arr[1]=false
  arr.each_index do |x|
    if arr[x] && x>1
      (x+1..arr.length).each do |a|
        (arr[a] = false) if arr[a] && a % x ==0
      end
    end
  end
  return arr
end