input: Write a function to compress a given string by replacing all repeating characters with a single instance and its count.
  
  def compress(s):
      # Initialize results
      output = ""
  
      # Initialize count
      count = 1
  
      # Traverse the string
      for i in range(len(s)):
  
          # If the current character is same
          # as next one, increment its count
          if (i+1 < len(s) and s[i] == s[i+1]):
              count += 1
  
          else:
              # Append the count and character
              output += str(count) + s[i]
  
              # Reset the count
              count = 1
  
      return output
  
  assert compress("aabbbbbbcccc")=="2a6b4c"
  assert compress("aabbcc")=="2a2b2c"
  assert compress("aaabbbcc")=="3a3b2c"
  assert compress("abc")=="1a1b1c"
  assert compress("")==""
output: def compress(s):
      if not s:
          return ""
      output = ""
      count = 1
      for i in range(len(s) - 1):
          if s[i] == s[i + 1]:
              count += 1
          else:
              output += str(count) + s[i]
              count = 1
      output += str(count) + s[-1]
      return output