回溯算法

class Solution {
    Set set = new HashSet<>();
    public String[] permutation(String s) {
        helper(s.toCharArray(),new int[s.length()],new StringBuilder());
        return set.toArray(new String[0]);
    }
    //chars是选择列表， flags是已选择列表
    public void helper(char[] chars,int[] flag,StringBuilder sb){
        if(sb.length()==chars.length){    
            set.add(sb.toString());
            return;
        }
        for(int i =0;i