input: Write a function to find the longest common substring in two input strings. The function should return the longest substring that appears in both strings, or an empty string if there is no common substring.
  
  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("abcde", "bcd")=="bcd"
  assert longestCommonSubstring("hello", "world")==""
  assert longestCommonSubstring("abcdef", "defghi")=="def"
  assert longestCommonSubstring("python", "java")==""
  assert longestCommonSubstring("", "programming")==""
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