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

Example Input: YHLJZrFQ, aSLJZQuo
Example Output: LJZ

Example Input: pYmzRnGPQVPrqPApKTyvaO, tNGPQVPrqPVTh
Example Output: GPQVPrqP

Example Input: QJYnoeSgQsxTUWdeBQ, oRSSgQsxTUWdeGH
Example Output:
SgQsxTUWde