320. Generalized Abbreviation

A word's generalized abbreviation can be constructed by taking any number of non-overlapping and non-adjacent substrings and replacing them with their respective lengths.

• For example, "abcde" can be abbreviated into:
  • "a3e" ("bcd" turned into "3")
  • "1bcd1" ("a" and "e" both turned into "1")
  • "5" ("abcde" turned into "5")
  • "abcde" (no substrings replaced)
• However, these abbreviations are invalid:
  • "23" ("ab" turned into "2" and "cde" turned into "3") is invalid as the substrings chosen are adjacent.
  • "22de" ("ab" turned into "2" and "bc" turned into "2") is invalid as the substring chosen overlap.

Given a string word, return a list of all the possible generalized abbreviations of word. Return the answer in any order.

Example 1:

Input: word = "word"
Output: ["4","3d","2r1","2rd","1o2","1o1d","1or1","1ord","w3","w2d","w1r1","w1rd","wo2","wo1d","wor1","word"]

Example 2:

Input: word = "a"
Output: ["1","a"]

Constraints:

• 1 <= word.length <= 15
• word consists of only lowercase English letters.