Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

思路：简单的dfs题目，遍历所有情况即可。

注意事项：StringBuffer，而不是BufferString。 另外要注意String a[]={“”，“”} 可以静态初始化； StringBuffer的deleteCharAt可以删除指定位置字符；String访问字符，charAt

另外注意询问：当输入为1时，输出？ 0时输出？ 以及“”时输出？ 这个要考虑。。本题目的解答分别对应“”，“”，“”

1. String digits_map[] ={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
2. List<String> result = new ArrayList<String>();
3. StringBuffer cur = new StringBuffer();
4. public void helper(String digits,int start,int n) {
5. if(start == n) {
6. result.add(new String(cur));
7. return;
8. }
9. char cchar = digits.charAt(start);
10. String str = digits_map[cchar-'0'];
11. for(int i=0;i< str.length();i++) {
12. cur.append(str.charAt(i));
13. helper(digits,start+1,n);
14. cur.deleteCharAt(cur.length()-1);
15. }
16. }
17. public List<String> letterCombinations(String digits) {
18. if(digits==null) return result;
19. int dlen = digits.length();
20. if(dlen==0) {
21. result.add(digits);
22. return result;
23. }
24. helper(digits,0,dlen);
25. return result;
26. }