https://leetcode.com/problems/permutations-ii/

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] ]

解题 ：我们需要一个boolean数组来维护哪个值我们用过。当有重复元素和之前差不多，需要在加入list的时候检测是否和上一个一样，一样而且用过就跳过。

public List<List<Integer>> permuteUnique(int[] nums) {
    List<List<Integer>> list = new ArrayList<>();
    Arrays.sort(nums);
    backtrack(list, new ArrayList<>(), nums, new boolean[nums.length]);
    return list;
}

private void backtrack(List<List<Integer>> list, List<Integer> tempList, int [] nums, boolean [] used){
    if(tempList.size() == nums.length){
        list.add(new ArrayList<>(tempList));
    } else{
        for(int i = 0; i < nums.length; i++){
            if(used[i] || i > 0 && nums[i] == nums[i-1] && !used[i - 1]) continue;
            used[i] = true; 
            tempList.add(nums[i]);
            backtrack(list, tempList, nums, used);
            used[i] = false; 
            tempList.remove(tempList.size() - 1);
        }
    }
}