code: def max_common_sub_string(s1, s2):
      # Assuming s2 has all unique chars
      s2dic = {s2[i]: i for i in range(len(s2))}
      maxr = 0
      subs = ""
      i = 0
      while i < len(s1):
          if s1[i] in s2dic:
              j = s2dic[s1[i]]
              k = i
              while j < len(s2) and k < len(s1) and s1[k] == s2[j]:
                  k += 1
                  j += 1
              if k - i > maxr:
                  maxr = k - i
                  subs = s1[i:k]
              i = k
          else:
              i += 1
      return subs
  
label_desc: Use image node_id 1 for calling a global function with example usage: max_common_sub_string(s1, s2) and returns: subs