You are given two strings, word1 and word2.

A substring of word1 is considered valid if its characters can be rearranged such that word2 becomes a prefix of the rearranged substring.

Return the total number of valid substrings of word1.

Constraints:

• $1 \leq$ word1.length $\leq 10^6$

• $1 \leq$ word2.length $\leq 10^4$

• word1 and word2 consist only of lowercase English letters.

Now, let’s take a moment to make sure you’ve correctly understood the problem. The quiz below helps you check if you’re solving the correct problem:

We have a game for you to play. Rearrange the logical building blocks to develop a clearer understanding of how to solve this problem.

Implement your solution in the following coding playground.