input: Write a function to find the longest common substring in two strings. The function should return the longest common substring found or an empty string if no common substring is found.
  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", "zabcdf")=="bcd"
  assert longestCommonSubstring("java", "lava")=="ava"
  assert longestCommonSubstring("python", "typhon")=="phon"
  assert longestCommonSubstring("abc", "def")==""
  assert longestCommonSubstring("", "")==""
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