Shrink

#include <iostream>
#include <vector>
using namespace std;
int main() {
  int n;
  vector<int> a;
  cin >> n;
  for (int i = 0; i < n; i++)
    a.push_back(i);
  cout << "a contains: ";
  for (size_t i = 0; i < a.size(); i++)
    cout << a[i] << ' ';
  cout << endl;

  for (int i = 0; i < n - 1; i++)
    a[i] += a[i + 1];
  a.erase(a.end() - 1);
  cout << "a contains: ";
  for (size_t i = 0; i < a.size(); i++)
    cout << a[i] << ' ';
  cout << endl;
}