input: Write a function to find the longest common substring between two given strings.
  assert longest_common_substring("abcdjfgh", "abcdjfeeee") == "abcdjf"
  assert longest_common_substring("abcdefg", "xyzabc") == "abc"
  assert longest_common_substring("abc", "def") == ""
output: def longest_common_substring(s1, s2):
      m = len(s1)
      n = len(s2)
      result = 0
      end = 0
      length = [[0] * (n + 1) for _ in range(m + 1)]
  
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if s1[i - 1] == s2[j - 1]:
                  length[i][j] = length[i - 1][j - 1] + 1
                  if length[i][j] > result:
                      result = length[i][j]
                      end = i
              else:
                  length[i][j] = 0
  
      return s1[end - result: end]