VMware last coding problem

vmware.py

"""
Compress String

input:
   i j
  'a 1 2 c 9 b 5 6 c 1'

output:
  'a12b56c10'
  
Explanation:
The solution that had submitted I had line 26 flipped in my condition statement.
I had to put the length check first before checking if it is an alphabet letter.
"""

def compress_string(s):
  if not s:
    return s
  
  number = ""
  chars = {}
  j = 0
  for index, char in enumerate(s):

    if char.isalpha():
      j = index + 1
      # get integer
      while j < len(s) and not s[j].isalpha():
        number += s[j]
        j += 1

      num = int(number)
      # reset the integer
      number = ""
      
      if char in chars:
        chars[char] += num
      else:
        chars[char] = num
  
  s = ""
  letters = list(chars.keys())
  
  for letter in sorted(letters):
    s += letter + str(chars[letter])

  return s
  
compress_string('a12c9b56c1')