In this task, you are given two strings A, B. Find the longest common substring in the strings A and B.

Example Input: mmDhYqPJC, qThYqxW
Example Output: hYq

Example Input: GHdeqnuYnQ, zzQeqnutAZ
Example Output: eqnu

Example Input: CBtXUAvNhZxEFUMXXynWlz, EzvZfPhZxEFUMXXUzDgvFKEb
Example Output:
hZxEFUMXX