Question

I’ve been asked to write a permutation function that uses recursion. The only parameter of the function should be the string that I should find all the permutations of. The function should return a vector with all possible permutations. I know I can use next_permutation in STL Algorithms, but I’ve been asked not to.

I have the base case set up, and I know I need a for loop, but I’m not quite sure where to go from there. Can someone point me in the right direction?

vector <string> getPerm(string str)
{
    vector<string> v;
    if(w.length() <= 1)
    {
        v.push_back(str);
        return v;
    }
    else
    {
        for(int i = 0; i < str.size(); i++)
        {
            //Some code
        }
    }
}

Any help would be appreciated.

Answer 1

Imagine you already have the result of the previous iteration of your function, with returns all the permutations of the first n-1 elements of your string.

vector<string>& v_prev = getPerm(str.substr(0, str.length()-1));

Use this in the

//Some code

part of your code.

Another tip: use the 0-length string as the stop-condition of your recursion. You can construct the 1-lenght permutations recursively 😉

Here is the entire solution:

vector<string> getPerm(string str)
{
  vector<string> v; 
  if (str.empty())
  {
    v.push_back(string());
    return v;
  }
  else
  {
    vector<string>& v_prev = getPerm(str.substr(0, str.length()-1));
    for(int i = 0; i < v_prev.size(); i++)
    {
      for (int j = 0; j < v_prev[i].length() + 1; j++)
      {
        string p = v_prev[i];
        p.insert(j, str.substr(str.length() - 1, 1));
        v.push_back(p);
      }
    }
    return v;
  }
}