message: update remove-duplicate-letter.py
diff: # Time: O(n)
  # Space: O(k), k is size of the alphabet
  -import collections
  +from collections import Counter
  class Solution(object):
  @@ -10,9 +10,7 @@ class Solution(object):
  :type s: str
  :rtype: str
  """
  - remaining = collections.defaultdict(int)
  - for c in s:
  - remaining[c] += 1
  + remaining = Counter(s)
  in_stack, stk = set(), []
  for c in s:
  @@ -23,4 +21,3 @@ class Solution(object):
  in_stack.add(c)
  remaining[c] -= 1
  return "".join(stk)
  -
  