Language: C++

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

#define lli long long
#define all(i) i.begin(),i.end()
#define rall(i) i.rbegin(),i.rend()
#define rep0(i,j) for(int i=0;i<j;i++)
#define rep1(i,j) for(int i=1;i<=j;i++)
#define rep0d(i,j) for(int i=j-1;i>=0;i--)
#define MOD 1000000007
#define MAX (lli)1e12 
typedef pair<lli, lli> p; 
class Graph{
  int V;
  vector<list<p>> adj; 
  const int INF = (int)1e6;

  public: 
  Graph(lli V): adj(V){ 
    this->V = V; 
  } 

  void addEdge(int u, int v, int w){ 
    adj[u].push_back(make_pair(v, w)); 
  } 

  lli edgeNum(){
    lli sum=0;
    rep0(i,V)sum+=adj[i].size();
    return sum;
  }

  bool route(lli from, lli to){//vec[i]==j: j->iっていうもとに戻れる道がある
    vector<lli> vec(V, -1);
    dfs(1,from,vec);
    return vec[to] >= 0;
  }

  void dfs(lli ed, lli num, vector<lli>& vec){
    for(auto e: adj[num]){
      if(vec[e.first] != -1)continue;
      vec[e.first] = num;
      dfs(ed+1, e.first, vec);
    }
  }

  vector<int> bfs(int n){
    queue<int> q;
    vector<int> vec(V,-1);

    q.push(n);
    vec[n]=0;
    while(!q.empty()){
      int p=q.front();
      q.pop();
      for(auto e:adj[p]){
        if(vec[e.first]==-1){
          q.push(e.first);
          vec[e.first] = e.second + vec[p];
        }
      }
    }
    return vec;
  }

  vector<int> shortestPath(int src){ 
    priority_queue< p, vector <p> , greater<p> > pq; 
  
    vector<int> dist(V, INF); 
    pq.push(make_pair(0, src)); 
    dist[src] = 0; 
  
    while (!pq.empty()) { 
        int u = pq.top().second; 
        pq.pop(); 
  
        list<p>::iterator i; 
        for (auto i = adj[u].begin(); i != adj[u].end(); ++i) { 
            int v = i->first; 
            int weight = i->second; 
  
            if (dist[v] > dist[u] + weight) { 
                dist[v] = dist[u] + weight; 
                pq.push(make_pair(dist[v], v)); 
            } 
        } 
    } 
    for(int i=0;i<V;i++)if(dist[i]==INF)dist[i]=-1;
    return dist;
  } 

};
struct BIT{
  vector<lli> tab;//bit table
  BIT(int n):tab(n+1){
    rep0(i,n+1)tab[i]=0;
  }

  void add(int p, lli num){
    for(++p;p<tab.size();p+=p&-p)tab[p]+=num;
  }

  lli sum(int p){//from 0 to p
    lli ans=0;
    for(++p;p>0;p-=p&-p)ans+=tab[p];
    return ans;
  }

  void debug(){
    rep0(i,tab.size())cout << tab[i] << ' ';
    cout << endl;
  }
};
//refs: https://algo-logic.info/segment-tree/
struct SegmentTree{
  const lli INF = (lli)1e12;
  vector<lli> st, lazy;
  lli leaf;
  SegmentTree(lli n):st(n*4,INF), lazy(n*4,INF){
    lli x=1;
    while(x<n)x<<=1;
    leaf=x;
  }

  void eval(int k){
    if(lazy[k]==INF)return ;
    if(k<leaf-1){
      lazy[2*k+1]=lazy[k];
      lazy[2*k+2]=lazy[k];
    }
    st[k]=lazy[k];
    lazy[k]=INF;
  }

  void update(lli p, lli k){
    eval(k);
    p+=leaf-1;
    st[p]=k;
    while(p>0){
      p=(p-1)/2;
      st[p]=min(st[2*p+1], st[2*p+2]);
    }
  }

  lli query(lli a, lli b){
    return sub_query(a, b, 0, 0, leaf);
  }

  lli sub_query(lli a, lli b, lli k, lli l, lli r){
    eval(k);
    if(r<=a || b<=l)return INF;
    else if(a<=l && r<=b){
      return st[k];
    }else{
      lli vl = sub_query(a,b,k*2+1,l,(l+r)/2),
          vr = sub_query(a,b,k*2+2,(l+r)/2,r);
      return min(vl,vr);
    }
  }

  void set(int i, lli x){
    st[i+leaf-1]=x;
  }

  void build(){
    rep0d(i,leaf-1)st[i]=min(st[2*i+1], st[2*i+2]);
  }
  
  void debug(){
    rep0(i,st.size())cout << st[i] << ' ';
    cout << endl;
  }
};

template <typename X>
struct SegTree {
    int n;
    FX fx;
    const X ex;
    vector<X> dat;
    SegTree(int n_, FX fx_, X ex_) : n(), fx(fx_), ex(ex_), dat(n_ * 4, ex_) {
        int x = 1;
        while (n_ > x) {
            x *= 2;
        }
        n = x;
    }
 
    void set(int i, X x) { dat[i + n - 1] = x; }
    void build() {
        for (int k = n - 2; k >= 0; k--) dat[k] = fx(dat[2 * k + 1], dat[2 * k + 2]);
    }
 
    void update(int i, X x) {
        i += n - 1;
        dat[i] = x;
        while (i > 0) {
            i = (i - 1) / 2;  // parent
            dat[i] = fx(dat[i * 2 + 1], dat[i * 2 + 2]);
        }
    }
 
    X query(int a, int b) { return query_sub(a, b, 0, 0, n); }
    X query_sub(int a, int b, int k, int l, int r) {
        if (r <= a || b <= l) {
            return ex;
        } else if (a <= l && r <= b) {
            return dat[k];
        } else {
            X vl = query_sub(a, b, k * 2 + 1, l, (l + r) / 2);
            X vr = query_sub(a, b, k * 2 + 2, (l + r) / 2, r);
            return fx(vl, vr);
        }
    }
};
 
p operator-(const p & l,const p & r) {   
    return {l.first-r.first,l.second-r.second};                                    
}
bool cmpf(const pair<lli, lli> left, const pair<lli, lli> right) {
    return left.first < right.first;
}
bool cmps(const pair<lli, lli> left, const pair<lli, lli> right) {
    return left.second < right.second;
}
 
void dfs(lli ed, lli num, vector<map<lli,lli>>& vec){
    lli n=1;
    for(auto e: vec[num]){
        if(n == ed)n++;
        if(e.second != -1)continue;
        vec[num][e.first] = n;
        vec[e.first][num] = n;   
        dfs(n, e.first, vec);
        n++;
    }
}
 
lli gcd(lli a, lli b){
    if(a < b)return gcd(b, a);
    lli temp;
    while((temp = a%b)){
        a=b;
        b=temp;
    }
    return b;
}

struct nCr{
  vector<lli> f, rf;
  lli count=0;
  nCr(lli num): f(num), rf(num){
      count = num;
      f[0] = 1;
      rep1(i, num) f[i] = (f[i - 1] * i) % MOD;
      rep0(i, num+1) rf[i] = inv(f[i]);
  }
    
  lli inv(lli x) {
      lli res = 1;
      lli k = MOD - 2;
      lli y = x;
      while (k) {
          if (k & 1) res = (res * y) % MOD;
          y = (y * y) % MOD;
          k /= 2;
      }
      return res;
  }
  lli C(lli n, lli k) {
      lli a = f[n];
      lli b = rf[n-k];
      lli c = rf[k];
   
      lli bc = (b * c) % MOD;
   
      return (a * bc) % MOD;
  }
};

struct UF{
    vector<int> par,size;

    UF(int N) : par(N), size(N, 1){
        rep0(i,N)par[i] = i;
    }

    int root(int x){
        if (par[x] == x) return x;
        return par[x] = root(par[x]);
    }

    void unite(int x, int y){
        int rx = root(x);
        int ry = root(y);
        if(rx == ry) return;
        par[rx] = ry;
        size[rx] += size[ry];
    }

    bool same(int x, int y){
        int rx = root(x);
        int ry = root(y);
        return rx == ry;
    }

    int getSize(int x){
        return size[x];
    }
};

void dfs(string str, int n){
  if(str.length()==n){
    cout << str << '\n';
  }else{
    rep0(i,(int)(str.back()-'a'+2)){
      dfs(str+(char)('a'+i), n);
    }
  }

}
int main(){
  int n;
  cin >> n;

  dfs("a",n);
}
  

Compiler output:
a.cc:178:5: error: 'FX' does not name a type; did you mean 'X'?
  178 |     FX fx;
      |     ^~
      |     X
a.cc:181:21: error: 'FX' has not been declared
  181 |     SegTree(int n_, FX fx_, X ex_) : n(), fx(fx_), ex(ex_), dat(n_ * 4, ex_) {
      |                     ^~
a.cc: In constructor 'SegTree<X>::SegTree(int, int, X)':
a.cc:181:43: error: class 'SegTree<X>' does not have any field named 'fx'
  181 |     SegTree(int n_, FX fx_, X ex_) : n(), fx(fx_), ex(ex_), dat(n_ * 4, ex_) {
      |                                           ^~