Java Solution By Recursion

[boyzhang]

class Solution {
    public String[] str = {"","","abc", "def","ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    public List<String> letterCombinations(String digits) {
        List<String> list = new ArrayList();
        String s = "";
        if(get(digits, 0, s, list) == 1) //
            return list;
        return new ArrayList();
    }
    public int get(String digits, int i, String s, List<String> list){ //此函数将实现 自 i之后的数字串的list
        if(digits.length() - i == 0) return 0;  //flag
        if(digits.length() - i == 1) {
            char ch = digits.charAt(i);
            int k = ch - '0';
            if(str[k].length() == 0) return 0;   //flag
            else{
                for(int j = 0; j < str[k].length(); j++){
                    String s2 = s;
                    s2 += str[k].charAt(j);
                    list.add(s2);
                }
            }
        }
        else{
            char ch = digits.charAt(i);
            int k = ch - '0';
            if(str[k].length() == 0) return 0;  //flag
            else{
                for(int j = 0; j < str[k].length(); j++){
                    String s2 = s;
                    s2 += str[k].charAt(j);
                    if(get(digits, i+1, s2, list) == 0)
                        return 0;
                }
            }
        }
        return 1;
    }
}