input: Write a python function to find the longest common substring between two strings.
  assert longest_substring("abcdef", "zabcf") == "abc"
  assert longest_substring("12345", "54321") == "1"
  assert longest_substring("hello world", "world hello") == " world"
  assert longest_substring("abcdxyz", "xyzabcd") == "abcd"
  assert longest_substring("banana", "ananas") == "anana"
output: def longest_substring(s1, s2):
      m, n = len(s1), len(s2)
      dp = [[0] * (n + 1) for _ in range(m + 1)]
      length = 0
      end_index = 0
  
      for i in range(1, m + 1):
          for j in range(1, n + 1):
              if s1[i - 1] == s2[j - 1]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
                  if dp[i][j] > length:
                      length = dp[i][j]
                      end_index = i
              else:
                  dp[i][j] = 0
  
      return s1[end_index - length:end_index]