Input: [1,2,3]
Output:
[
  [1,2,3],
  [1,3,2],
  [2,1,3],
  [2,3,1],
  [3,1,2],
  [3,2,1]
]

class Solution {
vector<vector<int>>res;
void traceback(int n, int step)
{
    if(step == n) return;
    int len=res.size();
    for(int i=0;i<len;i++)
    {
        for(int j=step+1;j<n;j++)
        {
            vector<int> tmp(res[i]);
        swap(res[i][step],res[i][j]);
        res.push_back(tmp);
        }
    }
    traceback(n,step+1);
}
public:
    vector<vector<int>> permute(vector<int>& nums) {
        res.push_back(nums);
        traceback(nums.size(), 0);
        return res;
    }
};