17,Letter Combinations of a Phone Number

Given a digit string, return all possible letter combinations that the number could represent.//给一个字符串，求出所以数字序列

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

题目分析：

解法很简单，就是递归，直接上代码。

代码如下：

public class Solution {private String[] numToChar={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};public void getAll(String digits,int pos,StringBuilder builder,List<String> list){//判断是否应该输出if(pos>=digits.length()){list.add(builder.toString());return;}//获得当前数字int num=digits.charAt(pos)-'0';for(int i=0;i<numToChar[num].length();++i){//添加builder.append(numToChar[num].charAt(i));getAll(digits, pos+1, builder, list);//删除builder.deleteCharAt(builder.length()-1);}return;}    public List<String> letterCombinations(String digits) {            List<String> list=new LinkedList<String>();        if(digits==null||digits.length()<1)    return list;        StringBuilder builder=new StringBuilder();        getAll(digits, 0, builder, list);        return list;    }}