题解 | 字符串的排列

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param str string字符串 
     * @return string字符串ArrayList
     */
    ArrayList<String> ans = new ArrayList<>();
    StringBuffer path = new StringBuffer();
    boolean[] used;
    public ArrayList<String> Permutation (String str) {
        // write code here
        if(str.length() < 2){
            ans.add(str);
            return ans;
        }
        used = new boolean[str.length()];
        back_tracking(str);

        return ans;
    }
    public void back_tracking(String str){
        if(path.length() == str.length()){
            ans.add(path.toString());
        }
        ArrayList<Character> chosed = new ArrayList<>();
        for(int i = 0; i < str.length(); i++){
            if(!used[i] && (chosed.isEmpty() || !chosed.contains(str.charAt(i)))){
                chosed.add(str.charAt(i));
                used[i] = true;
                path.append(str.charAt(i));
                back_tracking(str);
                path.deleteCharAt(path.length() - 1);
                used[i] = false;
            }
        }
    }
}

用一个list保存已经出现过的字符。