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: vhZuXCsIkQR, YflCuXCsIkLu
Example Output: uXCsIk

Example Input: bNrItRSJYlbKBslNhApOhI, rLWtbtRSJYlbKBsnWUCOK
Example Output:
tRSJYlbKBs