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]]

这道题和46. Permutations唯一的区别是，这道题给出的数组可能有重复数字，因此要剔除答案中的重复元素。最简单的解决方法是用set来存储答案。

class Solution {public:    vector<vector<int>> permuteUnique(vector<int>& nums) {        set<vector<int>> ans;        vector<int> out;        vector<int> visited(nums.size(), 0);        permuteUniqueDFS(ans, 0, nums, visited, out);        return vector<vector<int>>(ans.begin(), ans.end());    }    void permuteUniqueDFS(set<vector<int>>& ans, int level, vector<int>& nums, vector<int>& visited, vector<int>& out) {        if (level == nums.size()) ans.insert(out);        else {            for (int i = 0; i < nums.size(); i++) {                if (visited[i] == 0) {                    out.push_back(nums[i]);                    visited[i] = 1;                    permuteUniqueDFS(ans, level + 1, nums, visited, out);                    out.pop_back();                    visited[i] = 0;                }            }        }    }};