add "longest common subsequence in the given strings" problem

[dhnanj2]

The problem is standard-classical DP problem to determine the length of the longest common subsequence in the given pair strings.
know more about the problem here : https://www.geeksforgeeks.org/longest-common-subsequence-dp-4/
P.S. : I have read the contribution guidelines and would like to contribute and close this issue
P.S. : I searched for the above problem in DP section but later found it in strings section, I'm sorry for that.