利用回溯算法解决字符串的排列问题

import java.util.*;
public class Solution {
    ArrayList<String> res = new ArrayList<>();
    TreeSet<String> paths = new TreeSet<>();
    StringBuilder path = new StringBuilder();
    boolean [] visited;
    public ArrayList<String> Permutation(String str) {
        if(str == null || str.equals("")){
            return res;
        }   
        char[] strs = str.toCharArray();
        Arrays.sort(strs);
        visited = new boolean[strs.length];
        dfs(strs, 0);
        res.addAll(paths);
        return res;
    }
    //回溯框架
    public void dfs(char[] strs, int len){
        if(len == strs.length){
            paths.add(path.toString());
            return;
        }
        for(int i = 0; i < strs.length; i++){
            if(!visited[i]){
                visited[i] = true;
                path.append(strs[i]);
                dfs(strs, len+1);
                visited[i] = false;
                path.deleteCharAt(path.length()-1);
            }
        }

    }

}