I found an interesting problem on https://leetcode.com/problems/scramble-string/:
Given a string s1, we may represent it as a binary tree by partitioning it to two non-empty substrings recursively.

Below is one possible representation of s1 = "great":


    great
   /    \
  gr    eat
 / \    /  \
g   r  e   at
           / \
          a   t


To scramble the string, we may choose any non-leaf node and swap its two children.

For example, if we choose the node "gr" and swap its two children, it produces a scrambled string "rgeat".


    rgeat
   /    \
  rg    eat
 / \    /  \
r   g  e   at
           / \
          a   t


We say that "rgeat" is a scrambled string of "great".

Similarly, if we continue to swap the children of nodes "eat" and "at", it produces a scrambled string "rgtae".


    rgtae
   /    \
  rg    tae
 / \    /  \
r   g  ta  e
       / \
      t   a


We say that "rgtae" is a scrambled string of "great".

Given two strings s1 and s2 of the same length, determine if s2 is a scrambled string of s1.

Example 1:


Input: s1 = "great", s2 = "rgeat"
Output: true


Example 2:


Input: s1 = "abcde", s2 = "caebd"
Output: false

I tried it in Python, but could not do it. Can you solve it?
class Solution:
     def isScramble(self, s1, s2):
         if sorted(s1) != sorted(s2): return False
         if len(s1) < 4 or s1 == s2: return True
         f = self.isScramble
         for i in range(1, len(s1)):
             if f(s1[:i], s2[:i]) and f(s1[i:], s2[i:]) or f(s1[:i], s2[-i:]) and f(s1[i:], s2[:len(s1) - i]):
                 return True
         return False