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

Example Input: faHRBWyXsYzYmHQyy, aHTyXsYzYeXBFV
Example Output: yXsYzY

Example Input: IGHhMfqLeBpnullsPInfBH, LTDBpnullsPIBynwCrCQ
Example Output: BpnullsPI

Example Input: XjbqaKV, BZsbqajS
Example Output:
bqa