全排列

class Solution {
public:
    vector<vector<int>> permute(vector<int>& nums) {
        if(nums.empty())
            return {};
        
        permuteCore(0,nums);
        return res;    
    }
private:
    vector<vector<int>> res;
    
    void permuteCore(int index,vector<int> &nums)
    {
        if(index==nums.size()-1)
        {
            res.push_back(nums);
            return ;
        }
        
        for(int i=index;i<nums.size();++i)
        {
            swap(nums[index],nums[i]);
            permuteCore(index+1,nums);
            swap(nums[index],nums[i]);
        }
    }
};