Longest common substring
Input: S1 = "ABCDGH", S2 = "ACDGHR"
Output: 4
Explanation: The longest common substring
is "CDGH" which has length 4.
Example 2:
Input: S1 = "ABC", S2 "ACB"
Output: 1
Explanation: The longest common substrings
are "A", "B", "C" all having length 1.