This is an advanced DP problem. However once understood the logic, the coding becomes fairly simple. The most important thing is to understand the states of the DP. Here we are considering `4-D DP`.

You are given a 0-indexed string text and another 0-indexed string pattern of length 2, both of which consist of only lowercase English letters. You can add either pattern[0] or pattern[1] anywhere in ...

Abstract: We consider the problem of finding the number of subsequences when deleting symbols from a string. We present a framework to find closed-form expressions for the number of subsequences, and ...