old_contents: class Solution(object):
      def findTheDifference(self, s, t):
          """
          :type s: str
          :type t: str
          :rtype: str
          """
          chrs = {}
          res = 0
          for w in t:
              if w not in chrs:
                  chrs[w] = 1 << (ord(w) - 97)
              res += chrs[w]
          for w in s:
              res -= chrs[w]
          return chr(len(bin(res)) + 94)
  
new_contents: class Solution(object):
      def findTheDifference(self, s, t):
          """
          :type s: str
          :type t: str
          :rtype: str
          """
          chrs = {}
          res = 0
          for w in t:
              if w not in chrs:
                  chrs[w] = 1 << (ord(w) - 97)
              res += chrs[w] # it can be overflow
          for w in s:
              res -= chrs[w]
          return chr(len(bin(res)) + 94)
  
subject: Comment overflow problem on FindtheDiff_BitHash