• Write a function set < vector < int > > permute(vector < int > l, vector< int > post) which permutes the values in l and appends to the end of them the values of post. This is the difficult part: if you return such a set, then it is _already_ sorted for you, it is simply a matter of counting n elements into it.
  
  This function must be recursive (and function that runs in O(n!), or even O(2ⁿ) is going to be recursive.)
  
  The base case is if l is empty: just return the set containing post.
  
  OTHERWISE, loop through the elements of l, remove the current element from l, add it to the end of post, call permute, and add the results to the return value. It is best to make temp copies of l and post to not cause trouble. Might even make i and post const.
• If you really can't figure out how to do this, try looking at the same code written in Python. Understand the algorithm, and then deal with the STL separately.