OJ---有重复字符串的排列组合

class Solution {
    LinkedList<String> list = new LinkedList<>();
    public String[] permutation(String S) {
        dfs(S.toCharArray(),0);
        return list.toArray(new String[0]);
    }
    public void dfs(char[] c,int k){
        if(k==c.length){
            list.add(new String(c));
            return;
        }
        HashSet<Character> set = new HashSet<>();
        for(int i=k;i<c.length;i++){
            if(!set.contains(c[i])){
                set.add(c[i]);
                swap(c,i,k);
                dfs(c,k+1);
                swap(c,i,k);
            }
        }
    }
    public char[] swap(char[] c,int a,int b){
        char temp = c[a];
        c[a] = c[b];
        c[b] = temp;
        return c;
    }
}