JZ27 字符串的排列

排列问题
for循环 每次选一个字符 然后递归

import java.util.*;
public class Solution {
    ArrayList<String> res = new ArrayList<>();
    char[] c;
    public ArrayList<String> Permutation(String str) {
        c = str.toCharArray();
        dfs(0);
        return res;
    }
    public void dfs(int x){
        if(x == c.length-1) {res.add(new String(c)); return;}
        HashSet<Character> set = new HashSet<>();
        for(int i=x;i<c.length;i++){
            if(!set.contains(c[i])){
                set.add(c[i]);
                swap(i,x);
                dfs(x+1);
                swap(i,x);
            }
        }
    }
    public void swap(int i, int j){
        char temp = c[i];
        c[i]=c[j];
        c[j]=temp;
    }

}