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`.

> 原文:[https://www . geesforgeks . org/count-subseries-first-string-anagrams-second-string/](https://www.geeksforgeeks.org/count-subsequences-first-string-anagrams ...

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 ...

For k ∈ ℤ+, define Σ k as the set of integers {0, 1, …, k - 1}. Given an integer n and a string t of length m ≥ n over Σ k , we count the number of times that each one of the kn distinct strings of ...