题解 | #字符串的排列#

直接套回溯模板，不容易出错，一遍过
用一个Set过滤下重复就可以了

import java.util.*;
public class Solution {
    Set<String> res = new HashSet();
    public ArrayList<String> Permutation(String str) {
        boolean[] flag = new boolean[str.length()];
        back_track(str, "", flag);
        ArrayList<String> list = new ArrayList();
        for(String s : res){
            list.add(s);
        }

        return list;

    }

    public  void back_track(String str, String now, boolean[] flag) {
       if(now.length() == str.length()){
           res.add(now);
           return; 
       }
        // for 选择 in xuan'ze'lie'b
        for(int i = 0;i<str.length();i++ ){
            if( flag[i] ){
                continue;
            }else{
                // 做选择
                now += str.charAt(i);
                flag[i] = true;
                // 回溯 
                back_track(str, now, flag) ;
                // 撤销选择
                flag[i] = false;
                now = now.substring(0, now.length()-1);
            }
        }

    }
}