How can I reverse a series of numbers as quickly as possible? - vector

Question

I am trying to solve a question called 'smurf' on codebreaker.xyz, but my code is too slow. The task is to take in a series of numbers, one at a time, and reverse the array each time.
So for this input:
6
0 1 2 1 2 0
it should give the output 0 1 1 0 2 2.
The logic is correct and the code works, but how can I make it execute under 1 second for s <= 200,000...
This is the code in C++17:
#include <algorithm>
#include <bits/stdc++.h>
using namespace std;
#define int unsigned int
vector<int> S;
int s, t;
int32_t main(){
ios_base :: sync_with_stdio(false); cin.tie(0); cout.tie(0);
cin >> s;
for(int i = 0; i < s; i++){
cin >> t;
S.push_back(t);
reverse(S.begin(), S.end());
}
for(int j = 0; j < s; j++){
cout << S[j] << " ";
}
}

Answer 1

As I understand your question, you need to reverse your container every time you add a new element. We can omit that by just adding the new number to the back or the front of the container, depending on if the container should be "reversed" or not at the moment. Since adding elements to the front of a vector is quite costly I would suggest using a list instead. An example would look like this
#include <iostream>
#include <list>
#include <ranges> //c++20
int main() {
std::list<int> numbers{};
bool reversed = false;
int N;
std::cin >> N;
while (N>0) {
int inputNumber;
std::cin >> inputNumber;
if(reversed){
numbers.push_front(inputNumber);
}
else{
numbers.push_back(inputNumber);
}
reversed = not reversed;
--N;
}
if(reversed){
for(const int i : numbers | std::views::reverse){
std::cout << i << ' ';
}
}
else{
for(const int i : numbers){
std::cout << i << ' ';
}
}
}