寫這題的時候沒有啥DFS思路，感覺還是DFS沒刷明白，還需要多提高
參考鏈接：
【還得用回溯算法！| LeetCode：17.電話號碼的字母組合-嗶哩嗶哩】 https://b23.tv/oTuy71C

class Solution {public List<String> letterCombinations(String digits) {List<String> ans=new ArrayList<>();if(digits==null||digits.length()==0){return ans;    }char[] ch=digits.toCharArray();Map<Integer,String> map=new HashMap<>();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");StringBuilder sb=new StringBuilder();dfs(ch,map,ans,sb,0);return ans;}public void dfs(char[] ch,Map<Integer,String> map,List<String> ans,StringBuilder sb,int index){if(index==ch.length){ans.add(sb.toString());// sb.setLength(0);return;    }String curStr = map.get(ch[index]-'0');int n=curStr.length();for(int i=0;i<n;++i){sb.append(curStr.charAt(i));dfs(ch,map,ans,sb,index+1);sb.deleteCharAt(sb.length()-1);}}}