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]​​, and ​​[2,1,1]​​.


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​​Backtracking​​ 


class Solution {
public:
//先排序，以[1, 1, 2]，第一个1处理过以后，第二个1就可以跳掉了。
    vector<vector<int>> permuteUnique(vector<int>& nums) {
        
        int n=nums.size();
        vector< vector<int> >res;
        vector< int > path;
        
        sort(nums.begin(),nums.end());
        solve(res,nums,path,0,n);
        
        return res;
    }
    
    void solve(vector< vector<int> > &res, vector<int> &nums,vector<int> &path, int index, int n)
    {
        if(index==n)
        {
            res.push_back(path);
            return ;
        }
        
        for(int i=index;i<n;i++)
        {
            if(i>index && nums[i]==nums[index])//相等 不考虑
                continue;
            else
                swap(nums[i],nums[index]);
                
            path.push_back(nums[index]);
            solve(res,nums,path,index+1,n);
            path.pop_back();
        }
        sort(nums.begin()+index,nums.end());
    }
};