47. Permutations II

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Given a collection of numbers that might contain duplicates, return all possible unique permutations.

For example,
[1,1,2] have the following unique permutations:
[
[1,1,2],
[1,2,1],
[2,1,1]
]
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public class Solution {    public List<List<Integer>> permuteUnique(int[] nums) {        List<List<Integer>> rst = new ArrayList<List<Integer>>();        if (nums == null || nums.length == 0) return rst;        List<Integer> res = new ArrayList<>();        boolean[] used = new boolean[nums.length];        Arrays.sort(nums);        backtrace(nums, rst, res, used);        return rst;    }    public void backtrace(int[] nums, List<List<Integer>> rst, List<Integer> res, boolean[] used) {        if (res.size() == nums.length) {            rst.add(new ArrayList<>(res));            return;        }        for (int i = 0; i < nums.length; i++) {            if (used[i]) continue;            else if (i > 0 && nums[i] == nums[i-1] && used[i-1]) continue;            else {                used[i] = true;                res.add(nums[i]);                backtrace(nums, rst, res, used);                used[i] = false;                res.remove(res.size()-1);            }        }    }}

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