Letter Combinations of a Phone Number(Medium)

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.

解题思路：回溯+DFS

首先建一个表，来映射号码和字母的关系。然后对号码进行深度优先搜索，对于每一位，从表中找出数字对应的字母，这些字母就是本轮搜索的几种可能。

List<String> res;

public List<String> letterCombinations(String digits) {
    // 建立映射表
    String[] table = {" ", " ", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
    StringBuilder tmp = new StringBuilder();
    res = new LinkedList<String>();
    helper(table, 0, tmp, digits);
    return res;
}

private void helper(String[] table, int idx, StringBuilder tmp, String digits){
    if(idx == digits.length()){
        // 找到一种结果，加入列表中
        if(tmp.length()!=0) res.add(tmp.toString());
    } else {
        // 找出当前位数字对应可能的字母
        String candidates = table[digits.charAt(idx) - '0'];
        // 对每个可能字母进行搜索
        for(int i = 0; i < candidates.length(); i++){
            tmp.append(candidates.charAt(i));
            helper(table, idx+1, tmp, digits);
            tmp.deleteCharAt(tmp.length()-1);
        }
    }
}