17. Letter Combinations of a Phone Number

Letter Combinations of a Phone Number

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);
}
}
}