穷举vs暴搜vs深搜vs回溯vs剪枝系列一＞电话号码的字母组合

private String[] hash = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
    private StringBuffer path;
    private List<String> ret;
    public List<String> letterCombinations(String digits) {
        ret = new ArrayList<>();
        path = new StringBuffer();
        if(digits.length() == 0) return ret;

        dfs(digits,0);
        return ret;
    } 

    private void dfs(String digits,int pos){

        if(digits.length() == pos){
            ret.add(path.toString());
            return;
        } 

        char[] cur = hash[digits.charAt(pos)-'0'].toCharArray();
        for(int i = 0; i < cur.length; i++){
            path.append(cur[i]);
            dfs(digits, pos+1);

        //恢复现场 
        //deleteCharAt (int指数) 移除此序列中指定位置的字符
            path.deleteCharAt(path.length()-1);
        }
        
    }