input: Write a function to find the longest common substring in two given strings. The function should return the longest substring that appears in both strings. If the given strings are empty, the function should return an empty string.
  
  def longestCommonSubstring(string1, string2):
      n1=len(string1)
      n2=len(string2)
      
      L=[[0 for _ in range(n2+1)]for _ in range(n1+1)]
      longestSubstring=""
      longestSubstringLength=0
      for i in range(1, n1+1):
          for j in range(1, n2+1):
              if string1[i-1]==string2[j-1]:
                  L[i][j]=L[i-1][j-1]+1
                  if L[i][j]>longestSubstringLength:
                      longestSubstringLength=L[i][j]
                      longestSubstring=string1[i-1-longestSubstringLength+1:i]
              else:
                  L[i][j]=0
      return longestSubstring
  
  assert longestCommonSubstring("abcdef", "zbcdf")=="bcd"
  assert longestCommonSubstring("hello", "world")=="l"
  assert longestCommonSubstring("quick", "brown")==""
  assert longestCommonSubstring("", "")==""
  assert longestCommonSubstring("python", "thon")=="thon"
output: def longestCommonSubstring(string1, string2):
      n1=len(string1)
      n2=len(string2)
      
      L=[[0 for _ in range(n2+1)]for _ in range(n1+1)]
      longestSubstring=""
      longestSubstringLength=0
      for i in range(1, n1+1):
          for j in range(1, n2+1):
              if string1[i-1]==string2[j-1]:
                  L[i][j]=L[i-1][j-1]+1
                  if L[i][j]>longestSubstringLength:
                      longestSubstringLength=L[i][j]
                      longestSubstring=string1[i-1-longestSubstringLength+1:i]
              else:
                  L[i][j]=0
      return longestSubstring