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

Example Input: IGHhMfqLeBpnullsPInfBH, LTDBpnullsPIBynwCrCQ
Example Output: BpnullsPI

Example Input: PzXzvmYrlGZgOBk, iKnzvmYrlGZgOrs
Example Output: zvmYrlGZgO

Example Input: bzSLGjeqNBnSgVmhDq, xSgqNBnSgVzd
Example Output:
qNBnSgV