Use the copy_backward algorithms: Shift it right by 2 positions : copy_backward « STL Algorithms Non modifying sequence operations « C++ Tutorial

1. C++ Tutorial
2. STL Algorithms Non modifying sequence operations
3. copy_backward

#include <iostream>
#include <cassert>
#include <algorithm>
#include <vector>
#include <string>
#include <iostream>
using namespace std;

int main()
{
  string s("abcdefghihklmnopqrstuvwxyz");

  vector<char> vector1(s.begin(), s.end());

  copy_backward(vector1.begin(), vector1.end() - 2, vector1.end());
  vector<char>::iterator pos;

  for (pos=vector1.begin(); pos!=vector1.end(); ++pos) {
        cout << *pos << ' ';
  }

  return 0;
}

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