47. 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]
]

思路：大绝招

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

    public void traceback(List<List<Integer>> list,List<Integer> temp,int[] nums, boolean[] used){
        if (temp.size()==nums.length){
            list.add(new ArrayList<Integer>(temp));
        }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;
                temp.add(nums[i]);
                traceback(list,temp,nums,used);
                used[i] = false;
                temp.remove(temp.size()-1);
            }
        }

    }
}