input: Write a python function to find the longest common substring between two strings.
  assert longestCommonSubstring("Python", "Java") == ""
  assert longestCommonSubstring("ababc", "abcba") == "ab"
  assert longestCommonSubstring("abcdef", "zcdemf") == "cde"
output: def longestCommonSubstring(s1, s2):
      m, n = len(s1), len(s2)
      longest, end_idx = 0, 0
      dp = [[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]:
                  dp[i][j] = dp[i - 1][j - 1] + 1
                  if dp[i][j] > longest:
                      longest = dp[i][j]
                      end_idx = i
  
      return s1[end_idx - longest:end_idx]