LeetCode 17 (the alphabet of the phone number)

Idea: Use hashmap to store numbers and their corresponding strings, put the strings in vlue into each layer, add them in a loop, and enter the next layer through depth-first search. The recursion ends when the length of StringBuilder is equal to the length of digits.

class Solution {
    
    
    private  static List<String> list;
    private static Map<Character,String> map=new HashMap<>();
    private static  StringBuilder s=new StringBuilder();
    public List<String> letterCombinations(String digits) {
    
    
        map.put('2',"abc");
        map.put('3',"def");
        map.put('4',"ghi");
        map.put('5',"jkl");
        map.put('6',"mno");
        map.put('7',"pqrs");
        map.put('8',"tuv");
        map.put('9',"wxyz");
        list=new ArrayList<>();
        if (digits==null||digits.length()==0){
    
    
            return list;
        }
        DFS(digits,0);
        return list;
    }
    private void DFS(String digits,int index ){
    
    
        if (index==digits.length()){
    
    
            list.add(s.toString());
            return;
        }
        String s1=map.get(digits.charAt(index));
        for (char a:s1.toCharArray()){
    
    
            s.append(a);
            DFS(digits,index+1);
            s.deleteCharAt(s.length()-1);
        }
    }
}