The '''longest common subsequence''' (or [http://en.wikipedia.org/wiki/Longest_common_subsequence_problem '''LCS''']) of groups A and B is the longest group of elements from A and B that are common between the two groups and in the same order in each group. For example, the sequences "1234" and "1224533324" have an LCS of "1234":
'''1234'''
'''12'''245'''3'''332'''4'''
For a string example, consider the sequences "thisisatest" and "testing123testing". An LCS would be "tsitest":
'''t'''hi'''si'''sa'''test'''
'''t'''e'''s'''t'''i'''ng123'''test'''ing
In this puzzle, your code only needs to deal with strings. Write a function which returns an LCS of two strings (case-sensitive). You don't need to show multiple LCS's.
For more information on this problem please see [[wp:Longest_common_subsequence_problem|Wikipedia]].