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"]

要枚举，要么递归要么迭代，递归算法思路：每个字母都只有两种情况，保留或者缩写隐藏。用一个数记录缩写的字母数，代码如下，虽然看着topsulotion写出来，仿佛已经理解，但还是感觉自己写写不出来：

public class Solution {
    public List<String> generateAbbreviations(String word) {
        List<String> result = new ArrayList<String>();
        backtrack(result, new String(), word, 0, 0);
        return result;
    }
    public void backtrack(List<String> list, String str, String word, int start, int count) {
        if (start == word.length()) {
            if (count > 0) {
                str = str + count;
            }
            list.add(str);
        } else {
            backtrack(list, str, word, start + 1, count + 1);
            backtrack(list, str + (count > 0? count: "") + word.charAt(start), word, start + 1, 0);
        }
    }
}