Permutations

回溯。返回所有的全排列

    vector<vector<int> > permute(vector<int> &num) {
        // Note: The Solution object is instantiated only once and is reused by each test case.
        vector<vector<int>> res;
        vector<int> tmp;
        vector<bool> visited(num.size(),false);
        dfs(0,num,visited,tmp,res);
        return res;
    }
    
    void dfs(int cnt,vector<int> &num,vector<bool>& visited,vector<int>& tmp,vector<vector<int>>& res)
    {
        if(cnt==num.size())
        {
            res.push_back(tmp);
            return;
        }
        
        for(int i=0;i<num.size();i++)
        {
            if(!visited[i])
            {
                visited[i] = true;
                tmp.push_back(num[i]);
                dfs(cnt+1,num,visited,tmp,res);
                tmp.pop_back();
                visited[i] = false;
            }
        }
    }