input: Write a function to compress a given string by replacing all repeating characters with a single instance and its count. If the given string is empty then return an empty string.
  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 the compressed string
      return output
  
  assert compress("aabbbbbbcccc")=="2a6b4c"
  assert compress("aaabbbccc")=="3a3b3c"
  assert compress("abc")=="1a1b1c"
  assert compress("")==""
  assert compress("a")=="1a"
output: def compress(s): 
      if not s:
          return ""
      output = ""
      count = 1
      for i in range(1, len(s)):
          if s[i] == s[i-1]:
              count += 1
          else:
              output += str(count) + s[i-1]
              count = 1
      output += str(count) + s[-1]
      return output