leetcode17. Letter Combinations of a Phone Number

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"].

解法

回溯法
建立一个Character到char[]的map
当sb增加时，根据sb的长度选择要添加的key对应的字符，比如digits=“234”，比如sb的长度是1，digits.charAt(1)=3；sb的长度是0，digits.charAt(0)=2; map.get(digits.charAt(0)) ={‘a’, ‘b’, ‘c’}。依次添加的是a->d->g, a->d->h, a->d->i, a->e->g, a->e->h,….,c->f->i。

public class Solution {    public List<String> letterCombinations(String digits) {        List<String> ret = new ArrayList<>();        if (digits == null || digits.length() == 0) {            return ret;        }        Map<Character, char[]> map = new HashMap<Character, char[]>();        map.put('0', new char[]{});        map.put('1', new char[]{});        map.put('2', new char[]{'a', 'b', 'c'});        map.put('3', new char[]{'d', 'e', 'f'});        map.put('4', new char[]{'g', 'h', 'i'});        map.put('5', new char[] { 'j', 'k', 'l' });        map.put('6', new char[] { 'm', 'n', 'o' });        map.put('7', new char[] { 'p', 'q', 'r', 's' });        map.put('8', new char[] { 't', 'u', 'v'});        map.put('9', new char[] { 'w', 'x', 'y', 'z' });        StringBuilder sb = new StringBuilder();        helper(map, digits, sb, ret);        return ret;    }    private void helper(Map<Character, char[]> map, String digits, StringBuilder sb,                        List<String> result) {        if (sb.length() == digits.length()) {            result.add(sb.toString());            return;        }        for (char c : map.get(digits.charAt(sb.length()))) {            sb.append(c);            helper(map, digits, sb, result);            sb.deleteCharAt(sb.length() - 1);        }    }}