Scramble String

A nonempty string can be recursively split into two nonempty parts; at any split, the two children may either keep their order or swap. Determine whether second can be produced from first by repeating this operation.

Function
isScramble(first: String, second: String) → boolean

Examples
Example 1
first = "great"
second = "rgeat"
return = true
Case 1 exercises the documented deterministic contract.
Example 2
first = "abcde"
second = "caebd"
return = false
Case 2 exercises the documented deterministic contract.
Example 3
first = "a"
second = "a"
return = true
Case 3 exercises the documented deterministic contract.

Constraints
1 <= first.length == second.length <= 30.
Both strings contain lowercase English letters.

The trick is a recursive check with memoization on (i, j, len), where i is the start in first, j is the start in second, and len is the segment length. For each split k from 1 to len-1, test two cases. No swap: first[i..k] matches second[j..k] and the remainders match. Swap: first[i..k] matches the tail of second's segment, and the remainders match crosswise. Base case: equal substrings return true. Prune early by comparing character counts, since different multisets can never scramble into each other. The common pitfall is skipping the cache, which blows up exponentially. Another is mixing up the swapped indices, so write them out on paper first. If the index math slips under pressure, StealthCoder is the hedge during the live OA, because it reads the problem and gives you working code without the proctor seeing anything.

The honest play: practice the pattern, and have StealthCoder ready for the one you didn't see coming.