leetcode-电话号码的字母组合

class Solution {
    
     public List<String> letterCombinations(String digits) {
       List<String> list=new ArrayList();
         if(digits.length()==0)return list;
         String s="";
         backTrace(s,digits,0,list);
         return list;
     }
    void backTrace(String s,String digits,int len,List<String> list){
        String[] a={"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
        if(len==digits.length()){
            list.add(s);
           return;
        }
       String chars=a[digits.charAt(len)-‘0‘];
        for(int i=0;i<chars.length();i++){   
         //如果这里写的是s=s+chars.charAt(i)，而不是写在backTrace()中，会造成字符串增长，因为回溯的每一个结果都与其他结果无关
            backTrace(s+chars.charAt(i),digits,len+1,list);
        }
      
    }
}