Language: C++

 #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
 #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#include <iomanip>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define irep(it, stl) for(auto it = stl.begin(); it != stl.end(); it++)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << endl
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
int dx8[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy8[8] = {0, 1, 0, -1, 1, 1, -1, -1};
int dx4[4] = {1, 0, -1, 0};
int dy4[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
template<typename A, size_t N, typename T> void Fill(A (&array)[N], const T &val){ fill((T*)array, (T*)(array+N), val); }
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const int INF = INT_MAX;
const ll LLINF = 1LL<<60;
const ll MOD = 1000000007;
const double EPS = 1e-9;

//問題文が複雑な時は具体例を出して、小さい方から考えていく
//形だけが同じなら良くて、a~前の文字列で使った一番大きい文字+1が次の文字列で使える
//あとは形が同じでその中の辞書順最強をdfsで作っていく

int N;
string s = "";

void dfs(int v){
    //N文字になるまで
    if(v == N){
        fin(s);
    }
    
    
    //現在使える一番大きい文字
    char currentBigAlpha = "a";
    
    //sからbiggestを探す
    while(buf.find(currentBigAlpha) != string::npos){
        //aから初めてその文字列の一番大きいやつを探す
        currentBigAlpha++
    }
    //次で+1してるので--しておく
    currentBigAlpha--;

    for(char c="a";c<=currentBigAlpha+1;c++){
        s += c;
        //全列挙のときdfsを使うのはここでN=3だったら最初にaaaまでいってもどったときにsをpopして次にaabをして
        //その次はab→aba→abb→abcとやるから、popが便利！
        dfs(s.size());
        s.pop_back();
    }

}
int main(){

　　　cin >> N;
      dfs(0);
      
}

Compiler output:
a.cc:112:1: error: extended character 　 is not valid in an identifier
  112 | 　　　cin >> N;
      | ^
a.cc:112:1: error: extended character 　 is not valid in an identifier
a.cc:112:1: error: extended character 　 is not valid in an identifier
a.cc: In function 'void dfs(int)':
a.cc:91:28: error: invalid conversion from 'const char*' to 'char' [-fpermissive]
   91 |     char currentBigAlpha = "a";
      |                            ^~~
      |                            |
      |                            const char*
a.cc:94:11: error: 'buf' was not declared in this scope
   94 |     while(buf.find(currentBigAlpha) != string::npos){
      |           ^~~
a.cc:96:26: error: expected ';' before '}' token
   96 |         currentBigAlpha++
      |                          ^
      |                          ;
   97 |     }
      |     ~                     
a.cc:101:16: error: invalid conversion from 'const char*' to 'char' [-fpermissive]
  101 |     for(char c="a";c<=currentBigAlpha+1;c++){
      |                ^~~
      |                |
      |                const char*
a.cc: In function 'int main()':
a.cc:112:1: error: '\U00003000\U00003000\U00003000cin' was not declared in this scope
  112 | 　　　cin >> N;
      | ^~~~~~~~~