题解 | #字符串的排列#经典回溯排列问题

import java.util.ArrayList;
import java.util.HashSet;
public class Solution {
   ArrayList<String> result = new ArrayList<String>();
    HashSet<String> set = new HashSet<String>();

    StringBuilder sb = new StringBuilder();
    boolean[] used;

    public ArrayList<String> Permutation(String str) {
        if (str == null || str.length() == 0) {
            return result;
        }

        used = new boolean[str.length()];
        helper(str);
        for (String s : set) {
            result.add(s);
        }
        return result;
    }

    private void helper(String str) {
        if (sb.length() == str.length()) {
            set.add(new String(sb).toString());
            return;
        }
        for (int i = 0; i < str.length(); i++) {
            if (used[i]) {
                continue;
            }
            used[i] = true;
            sb.append(str.charAt(i));
            helper(str);
            sb.deleteCharAt(sb.length() - 1);
            used[i] = false;
        }

    }


}