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class Solution {

/**

* @param nums: A list of integers.

* @return: A list of permutations.

*/

public ArrayList<ArrayList<Integer>> permute(ArrayList<Integer> nums) {

// write your code here

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

if (nums == null || nums.size() == 0) {

return res;

}

int size = nums.size();

boolean[] used = new boolean[size];

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

for (int i = 0; i < size; i++) {

used[i] = true;

tempList.add(nums.get(i));

dfsHelper(res, nums, used, tempList);

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

used[i] = false;

}

return res;

}

public void dfsHelper(ArrayList<ArrayList<Integer>> res, ArrayList<Integer> nums, boolean[] used, ArrayList<Integer> tempList) {

if (tempList.size() == nums.size()) {

// valid permutation.

res.add(new ArrayList<Integer>(tempList));

return;

}

for (int i = 0; i < nums.size(); i++) {

if (used[i]) {

continue;

}

used[i] = true;

tempList.add(nums.get(i));

dfsHelper(res, nums, used, tempList);

used[i] = false;

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

}

}

}