17. Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

Input:Digit string “23”
Output: [“ad”, “ae”, “af”, “bd”, “be”, “bf”, “cd”, “ce”, “cf”].
使用回溯法：

public class Solution {    List<String> list; //需要全局的    public List<String> letterCombinations(String digits) {        list=new ArrayList<String>();        if(digits.length()==0) return list;        backTrack(digits,"",0);  //digits第一个数字        return list;    }    void backTrack(String digits,String letter,int index){        if(index>=digits.length()){            list.add(new StringBuilder(letter).toString());            return;        }        int num = digits.charAt(index) - '2';  //键盘数字、字母是从2开始的        int end;        if(num==5||num==7) end=4;  //数字7和9是四个字母        else end=3;        for(int i=0;i<end;i++){            char c;            if(num<5) c=(char)('a'+(num*3+i));            else if(num==5) c=(char)('p'+i);  //7            else if(num==6) c=(char)('t'+i);  //8            else c=(char)('w'+i);             //9            String newLetter = new StringBuilder(letter).toString();            backTrack(digits,letter+c,index+1);        }    }}