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], and [2,1,1].

class Solution {public:    void _swap (int &a, int &b) {        int tmp = a;        a = b;        b = tmp;    }    bool exist(vector
     
      & nums, int num, int start, int end) {        int i = 0;        for (i=start; i
      

> &res, vector

& nums, int start, int end) { if (start >= end) { res.push_back(nums); return; } int i; for (i=start; i<=end; i++) { if (exist(nums, nums[i], start, i)) { continue; } _swap(nums[start], nums[i]); _permute(res, nums, start+1, end); _swap(nums[start], nums[i]); } } vector

> permuteUnique(vector

& nums) { sort(nums.begin(), nums.end()); vector

> res; int start = 0; int end = nums.size() - 1; _permute(res, nums, start ,end); return res; }};