Problem
跟 3297. Count Substrings That Can Be Rearranged to Contain a String I 一模一样，只是多了一个注意点：
Note that the memory limits in this problem are smaller than usual, so you must implement a solution with a linear runtime complexity.
https://leetcode.cn/problems/count-substrings-that-can-be-rearranged-to-contain-a-string-ii/
Thoughts
Problem 3297 的时间复杂度 O(n + m)（n 和 m 分别是 word1 和 word2 的长度），空间复杂度 O(k) ≈ O(1)（k 是 word2 中不同字符的个数）。基本符合本题的限制，可以直接用。
