320. Generalized Abbreviation

Write a function to generate the generalized abbreviations of a word.

Example:
Given word ="word", return the following list (order does not matter):
["word", "1ord", "w1rd", "wo1d", "wor1", "2rd", "w2d", "wo2", "1o1d", "1or1", "w1r1", "1o2", "2r1", "3d", "w3", "4"]

好赞的递归方式， 膜拜， 还要仔细看一遍！