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public class Solution {

private HashMap<Integer, HashSet<Integer>> map;

private int[] num;

private boolean[] used;

private ArrayList<Integer> current;

private ArrayList<ArrayList<Integer>> answer;

public ArrayList<ArrayList<Integer>> permute(int[] num) {

map = new HashMap<Integer, HashSet<Integer>>();

answer = new ArrayList<ArrayList<Integer>>();

this.num = num;

used = new boolean[num.length];

current = new ArrayList<Integer>();

search(0);

return answer;

}

private void search(int p) {

if (p == num.length) {

ArrayList<Integer> newList = new ArrayList<Integer>();

for (Integer i : current) {

newList.add(i);

}

answer.add(newList);

return;

}

for (int i = 0; i < used.length; i++) {

if (!used[i]) {

HashSet<Integer> positions = map.get(num[i]);

if (positions == null || !positions.contains(p)) {

used[i] = true;

if (positions == null) {

positions = new HashSet<Integer>();

}

positions.add(p);

map.put(num[i], positions);

current.add(num[i]);

search(p+1);

current.remove(current.size() - 1);

positions.remove(p);

map.put(num[i], positions);

used[i] = false;

}

}

}

}

}