Got this question while interviewing for a startup (Alchemy), could not solve it in time. Would be great to get input from the community on an optimal solution.\n\n**Question:**\n\nGiven a string of only characters str, seperate the string into subsets such that each subset has the same single-digit total sum. Use an unserscore to seperate subsets in the input string. Return all such subsets creations in a result array, and then sort that array lexicographically before returning it.\n\n**Example and Explanation:**\n\n```\nInput: "1741380"\nOutput: ["174_413_80", "174_1380", "1741380"]\n```\n\n**Explanation:**\nThe last element in the result array "1741380" is just the input string itself. This is considered to be one of the subsets as the whole input is a subset of itself. Additionally, as this is the only subset, the sum of its string digits is 1+7+4+1+3+8+0 which is 24. With no other subset to compare to, this means that every subset (just the one) has the same single-digit sum. As for the other two:\n\n```\n1+7 -> 8\n4+1+3 -> 8\n8+0 -> 8\n\n1+7+4 -> 12\n1+3+8+8 -> 12\n```\n\n**Another example:**\n```\nInput: 1111\nOutput: ["1_1_1_1", "11_11", "1111"]\n```

Got this question while interviewing for a startup (Alchemy), could not solve it in time. Would be great to get input from the community on an optimal solution.