Leetcode 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], and [2,1,1].

Difficulty: Medium

回溯的基础上加一行判断。

</pre><pre name="code" class="cpp">class Solution {public:    void back_tracking(vector<vector<int> > &ans, vector<int> &temp, vector<int> &nums, vector<int> &flag){        if(temp.size() == nums.size()){            ans.push_back(temp);            return;        }                for(int i = 0; i < flag.size(); i++){            if(flag[i] == 1 || (i > 0 && nums[i] == nums[i-1] && flag[i-1] == 0))                continue;            temp.push_back(nums[i]);            flag[i] = 1;            back_tracking(ans, temp, nums, flag);            flag[i] = 0;            temp.pop_back();        }        return;            }    vector<vector<int>> permuteUnique(vector<int>& nums) {        sort(nums.begin(), nums.end());        vector<vector<int> > ans;        vector<int> temp;        vector<int> flag(nums.size(), 0);        back_tracking(ans, temp, nums, flag);        return ans;            }};

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