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

Example Input: jeQzwP, kSQzxq
Example Output: Qz

Example Input: XYnCjAWPHHJ, gmCjAWPHpmwVsm
Example Output: CjAWPH

Example Input: ksZZaowRzXsbwlaJrcWD, OuGFjxGFCZZaowRzXsgWrJzO
Example Output:
ZZaowRzXs