main.cpp

#include <iostream>
#include <vector>

using namespace std;

signed main() {
   vector<int> arr{0, 1, 2, 3, 4, 5};

   arr.push_back(6); // O(1)
   arr.pop_back(); // O(1)

   arr.insert(arr.begin() + 3, 6);

   return 0;
}