Function f

CSES/Intro/TowerH.cc:6–12

vector<pair<int,int>> ans;

void f(int n,int from,int to,int aux) {
    if (n == 0) return;
    cnt++;
    f(n-1,from,aux,to);
    ans.push_back({from,to});
    f(n-1,aux,to,from);
}

int solve(int n) {
    f(n,1,3,2);