#include <string>
#include <vector>

using namespace std;

/**
   Given a vector of integers, the altSum method
   should return the alternating sum
   v[0] - v[1] + v[2] - v[3] + ...
   If the vector is empty, the alternating sum is zero.
   Use a recursive helper method. Do not use loops.
*/

int altSumHelper(const vector<int>& values, int i)
{
   int size = values.size();
   if (size == 0) {
      return i;
   }

   if (size % 2 == 0) {
      i -= values.back();
   } else {
      i += values.back();
   }
   vector<int> temp = values;
   temp.pop_back();


   return (altSumHelper(temp, i));
}

int altSum(vector<int> v)
{
   return altSumHelper(v, 0);
}