### Longest Common Subsequence ###

Given two strings 'a' and 'b', find the length of their longest common subsequence.
A subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.

- For example, "ace" is a subsequence of "abcde".

A common subsequence of two strings is a subsequence that is common to both strings.