Letter Combinations of a Phone Number

题目

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.

方法

使用递归求解。

    char[][] letters = {{'a','b','c'},{'d','e','f'},{'g','h','i'},{'j','k','l'},{'m','n','o'},{'p','q','r','s'},{'t','u','v'},{'w','x','y','z'}};    public void getCombi(String digits, int cur, int len, String str, List<String> list) {        if (cur == len) {            list.add(str);        } else {            int index = digits.charAt(cur) - '2';            int leng  = letters[index].length;            for (int i = 0; i < leng; i++) {                String newStr = str +  letters[index][i];                getCombi(digits, cur + 1, len, newStr, list);            }                    }    }    public List<String> letterCombinations(String digits) {        List<String> list = new ArrayList<String>();        String str = "";        int len = digits.length();        getCombi(digits, 0, len, str, list);        return list;    }