输入2 个长度不大于250000的字符串，输出这2 个字符串的最长公共子串。如果没有公共子串则输出0 。
A string is finite sequence of characters over a non-empty finite set Σ.
In this problem, Σ is the set of lowercase letters.
Substring, also called factor, is a consecutive sequence of characters occurrences at least once in a string.
Now your task is simple, for two given strings, find the length of the longest common substring of them.
Here common substring means a substring of two or more strings.
The input contains exactly two lines, each line consists of no more than 250000 lowercase letters, representing a string.
The length of the longest common substring. If such string doesn’t exist, print “0” instead.