比較經典的回溯演算法，用了之後記得要標記出來。標記後下一輪記得重置

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        vector<vector<int>> res;
        if(nums.empty())
            return res;
        int n=nums.size();
        vector<int> out;
        bool *used=new bool[n];
        memset(used,0,n);
        shengcheng(res,nums,out,used,n);
        delete[] used;
        return res;
        
    }
    void shengcheng(vector<vector<int>>&res,vector<int>& nums,vector<int>&out,bool *used,int total)
    {
        if(out.size()==total)
        {
            res.push_back(out);
            //out.pop_back();
        }
        for(int i=0;i<total;++i)
        {
            if(!used[i])
            {
                out.push_back(nums[i]);
                used[i]=true;
                shengcheng(res,nums,out,used,total);
                used[i]=false;
                out.pop_back();
            }      
        }
    }
};