What is the fine-grained topic of the following text: You are given two strings s and t with respective lengths m and n. Tell if s is a subsequence of t, that is, if there is a subset of m positions of t, 0 ≤ j0 < … < jm−1 < n, such that s[i] = t[ji] for all 0 ≤ i < m. Additionally, tell if there is just one such subset of positions. Only return the topic.
Checking if a string is a subsequence of another string and finding the subset of positions.