old_contents: """
  Given an array and a value, remove all instances of that value in place and
  return the new length.
  
  Do not allocate extra space for another array, you must do this in place with
  constant memory.
  
  The order of elements can be changed. It doesn't matter what you leave beyond
  the new length.
  
  Example:
      Given input array nums = [3,2,2,3], val = 3
  
      Your function should return length = 2, with the first two elements of nums
      being 2.
  
  Hint:
  
      Try two pointers.
      Did you use the property of "the order of elements can be changed"?
      What happens when the elements to remove are rare?
  """
  class Solution(object):
      def removeElement(self, nums, val):
          """
          :type nums: List[int]
          :type val: int
          :rtype: int
          """
          if nums==[]:
              return 0
          i=0
          while True:
              if nums[i]==val:
                  del nums[i]
              else:
                  i+=1
              if i>=len(nums):
                  break
          return len(nums)
  
new_contents: """
  Given an array and a value, remove all instances of that value in place and
  return the new length.
  
  Do not allocate extra space for another array, you must do this in place with
  constant memory.
  
  The order of elements can be changed. It doesn't matter what you leave beyond
  the new length.
  
  Example:
      Given input array nums = [3,2,2,3], val = 3
  
      Your function should return length = 2, with the first two elements of nums
      being 2.
  
  Hint:
  
      Try two pointers.
      Did you use the property of "the order of elements can be changed"?
      What happens when the elements to remove are rare?
  """
  class Solution(object):
      def removeElement(self, nums, val):
          """
          :type nums: List[int]
          :type val: int
          :rtype: int
          """
          if nums is None or nums == []:
              return 0
          l = len(nums)
          left, right = 0, l - 1
          while right > 0 and nums[right] == val:
              right -= 1
          while left < right:
              if nums[left] == val:
                  nums[left], nums[right] = nums[right], nums[left]
                  while right > 0 and nums[right] == val:
                      right -= 1
              left += 1
          return left if nums[left] == val else left + 1
  
  a = Solution()
  print(a.removeElement([3], 3) == 0)
  print(a.removeElement([2], 3) == 1)
  print(a.removeElement([3, 2, 2, 3], 3) == 2)
  print(a.removeElement([3, 3, 2, 3], 3) == 1)
  print(a.removeElement([3, 2, 2, 3], 3) == 2)
  print(a.removeElement([3, 3, 3, 3], 3) == 0)
  
subject: revise 027 to be in-place