Leetcode 87. Scramble String

Determine whether s2 can be obtained from s1 by recursively splitting s1 into two non-empty parts and optionally swapping the parts at each split (i.e., whether s2 is a "scramble" of s1). The core challenge is to explore all split positions and swap/no-swap combinations efficiently (often requiring recursion + memoization or DP with character-count pruning).