How Can I Rewrite and Condense the Following C++ Code? [closed]

Currently I'm taking an introductory computer science class. My teacher has asked us to rewrite and condense the following code. I've been fruitlessly researching for days now. If someone might throw me a bone It'd be much appreciated.

#include <iostream>
#include <stratagm.h++>

p_data r0ll(p_key _p_key, p_data _p_data) {
  std::vector<char> vecS;
  std::vector<char> vecT;
  std::vector<char> vecK;

  unsigned short i, j, temp;

  std::cout << "\n @ 0\n" << std::flush;

  for (i = 0; i < 256; i++) {
    vecS.push_back(i);
    vecT.push_back(_p_key[i]);
  }

  for (i = 0; i < _p_data->size(); i++)
    vecK.push_back(_p_data->at(i));
  for (i = 0; i < 256; i++) {
    j = (j + vecS[i] + vecT[i]) % 256;
    temp = vecS[i];
    vecS[i] = vecS[j];
    vecS[j] = temp;
  }

  for (i = 0; i < 256; i++) {
    _p_data->at(i) = vecK.at(i) ^ vecT.at(i);
  }

  return _p_data;
}