17. 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.

DFS遍历即可。这类题型，DFS都可以求解。

class Solution {    public void dfs(String digits, HashMap<Integer, String> map, List<String> lst,                     StringBuilder s, int begin){        if ( begin == digits.length()){            lst.add(new String(s));        }        else {            String tmp = map.get(digits.charAt(begin) - '0');            for (int i = 0; i < tmp.length(); ++ i){                s.append(tmp.charAt(i));                dfs(digits, map, lst, s, begin + 1);                s.deleteCharAt(s.length() - 1);            }        }    }        public List<String> letterCombinations(String digits) {        HashMap<Integer, String> map = new HashMap<>();        map.put(0, "");        map.put(1, "");        map.put(2, "abc");        map.put(3, "def");        map.put(4, "ghi");        map.put(5, "jkl");        map.put(6, "mno");        map.put(7, "pqrs");        map.put(8, "tuv");        map.put(9, "wxyz");List<String> lst = new ArrayList<>();        if (digits.length() == 0){            return lst;        }        StringBuilder s = new StringBuilder();        dfs(digits, map, lst, s, 0);        return lst;    }}