input: Write a python function to find the largest common substring between two strings.
  assert largest_common_substring("killerbees", "yellow") == "ll"
  assert largest_common_substring("abcdef", "defabc") == "def"
  assert largest_common_substring("12345", "54321") == "1"
output: def largest_common_substring(str1, str2):
      m, n = len(str1), len(str2)
      max_len = 0
      ending_index = m
      
      length = [[0] * (n + 1) for _ in range(m + 1)]
      
      for i in range(m + 1):
          for j in range(n + 1):
              if i == 0 or j == 0:
                  length[i][j] = 0
              elif str1[i - 1] == str2[j - 1]:
                  length[i][j] = length[i - 1][j - 1] + 1
                  if length[i][j] > max_len:
                      max_len = length[i][j]
                      ending_index = i
              else:
                  length[i][j] = 0
  
      return str1[ending_index - max_len: ending_index]