Euler Totient Precompute

#include <bits/stdc++.h>
using namespace std;

const int maxN = 1e5;
int64_t phi[maxN];

void preComputePhi () {
    for (int i = 1; i < maxN; ++i) phi[i] = i;
    for (int i = 2; i < maxN; ++i) {
        if (phi[i] == i) {
            //i is a prime number
            //we need to subtract 1 from phi[i] because we know that phi[p] = p - 1
            for (int j = i; j < maxN; j += i) {
                phi[j] -= phi[j] / i;
            }
        }
    }
}

int main () {
     ios::sync_with_stdio(false);
     cin.tie(nullptr);
     cout.tie(nullptr);
     int T = 1;
     //~ cin >> T;
     preComputePhi();
     for (int test_case = 1; test_case <= T; ++test_case) {
         for (int i = 1; i < 30; ++i) {
             cout << phi[i] << ' ';
         }
     }
     //cerr << "Time elapsed :" << clock() * 1000.0 / CLOCKS_PER_SEC << " ms" << '\n';
}