Language: C++

// https://atcoder.jp/contests/sumitrust2019/tasks/sumitb2019_d
#define CIN_ONLY
#define DECIMAL_DIGITS 10
#define STATIC_MOD 1e9 + 7

#ifdef BTK
/*<head>*/
#    include "Template.hpp"
/*</head>*/
#else
/*<body>*/
/* #region auto includes */
/* #region stl */
/*<stl>*/
#    include <bits/stdc++.h>
#    include <sys/types.h>
#    include <unistd.h>
using namespace std;
/*</stl>*/
/* #endregion */
/* #region template/IncludeSTL.hpp*/
/**
 * @file IncludeSTL.hpp
 * @author btk
 * @brief 標準ライブラリをincludeするだけ
 * @version 0.1
 * @date 2019-07-21
 * @todo 何故か2回includeされる（展開scriptに
 * @copyright Copyright (c) 2019
 *
 */


/* #endregion */
/* #region template/Macro.hpp*/
/**
 * @file Macro.hpp
 * @author btk
 * @brief マクロとか，LLとか
 * @version 0.1
 * @date 2019-07-13
 *
 * @copyright Copyright (c) 2019
 *
 */


//! LL
using LL = long long;

/**
 * @def DEBUG
 * @brief デバッグ用のif文 提出時はif(0)で実行されない
 */

/*</head>*/
#    ifdef BTK
#        define DEBUG if (1)
#    else
#        ifdef CIN_ONLY
#            define FAST_IO
#        endif
#        define DEBUG if (0)
#    endif
/**
 * @def ALL(v)
 * @brief
 * ALLマクロ
 */
#    define ALL(v) (v).begin(), (v).end()

/**
 * @def REC(ret, ...)
 * @brief
 * 再帰ラムダをするためのマクロ
 */
#    define REC(ret, ...) std::function<ret(__VA_ARGS__)>

/**
 * @def VAR_NAME(var)
 * @brief 変数名を取得する
 */
#    define VAR_NAME(var) #    var

/**
 * @brief
 * rangeで生まれる使わない変数を消す用（警告消し）
 */
template <typename T>
inline T& unused_var(T& v) {
    return v;
}
/* #endregion */
/* #region template/IO.hpp*/
/**
 * @file IO.hpp
 * @author btk
 * @brief cin高速化とか，出力の小数桁固定とか
 * @version 0.1
 * @date 2019-07-13
 *
 * @copyright Copyright (c) 2019
 */


/**
 * @brief 入出力の設定を行うための構造体
 */
struct cww {
    /**
     * @brief Construct a new cww::cww object
     * @details
     * CIN_ONLYを定義すると，submit時にcinとscanfの同期を切る設定が走る
     * DECIMAL_DIGITSを定義すると，doubleの出力時指定した桁数分小数部を吐くようになる
     */
    cww() {
#    ifdef FAST_IO
        ios::sync_with_stdio(false);
        cin.tie(0);
#    endif
#    ifdef DECIMAL_DIGITS
        cout << fixed;
        cout << setprecision(DECIMAL_DIGITS);
#    endif
    }
};

//! 入出力設定構造体を実体化
cww star;

/**
 * @brief
 * vectorに直接cin流すためのやつ
 * @tparam T
 * @param is
 * @param v
 * @return istream&
 */
template <typename T>
std::istream& operator>>(std::istream& is, std::vector<T>& v) {
    for (auto& it : v) is >> it;
    return is;
}
/* #endregion */
/* #region template/Loop.hpp*/
/**
 * @file Loop.hpp
 * @author btk
 * @brief rangeとかループ系のクラス
 * @version 0.1
 * @date 2019-07-13
 *
 * @copyright Copyright (c) 2019
 *
 */


/**
 * @brief
 * rangeを逆向きに操作したいとき用
 * @details
 * ループの範囲は[bg,ed)なので注意
 * @see range
 */
class reverse_range {
  private:
    struct I {
        int x;
        int operator*() { return x - 1; }
        bool operator!=(I& lhs) { return x > lhs.x; }
        void operator++() { --x; }
    };
    I i, n;

  public:
    /**
     * @brief Construct a new reverse range object
     *
     * @param n
     */
    reverse_range(int n) : i({0}), n({n}) {}
    /**
     * @brief Construct a new reverse range object
     *
     * @param i
     * @param n
     */
    reverse_range(int i, int n) : i({i}), n({n}) {}
    /**
     * @brief
     * begin iterator
     * @return I&
     */
    I& begin() { return n; }
    /**
     * @brief
     * end iterator
     * @return I&
     */
    I& end() { return i; }
};
/**
 * @brief
 * python みたいな range-based for を実現
 * @details
 * ループの範囲は[bg,ed)なので注意
 * !つけると逆向きにループが回る (reverse_range)
 * 空間計算量はO(1)
 * 使わない変数ができて警告が出がちなので，unused_varとかを使って警告消しするとよい
 */
class range {
  private:
    struct I {
        int x;
        int operator*() { return x; }
        bool operator!=(I& lhs) { return x < lhs.x; }
        void operator++() { ++x; }
    };
    I i, n;

  public:
    /**
     * @brief Construct a new range object
     *
     * @param n
     */
    range(int n) : i({0}), n({n}) {}
    /**
     * @brief Construct a new range object
     *
     * @param i
     * @param n
     */
    range(int i, int n) : i({i}), n({n}) {}
    /**
     * @brief
     * begin iterator
     * @return I&
     */
    I& begin() { return i; }
    /**
     * @brief
     * end iterator
     * @return I&
     */
    I& end() { return n; }
    /**
     * @brief
     * 逆向きに参照するrange(reverse_rangeを取得できるs)
     * @return reverse_range
     */
    reverse_range operator!() { return reverse_range(*i, *n); }
};
/* #endregion */
/* #region template/MinMaxOperation.hpp*/
/**
 * @file MinMaxOperation.hpp
 * @author btk
 * @brief 最大値とか最小値を求める
 * @version 0.1
 * @date 2019-07-04
 *
 * @copyright Copyright (c) 2019
 *
 */

/**
 * @brief 2項の最小値求める
 *
 * @tparam T
 */
template <typename T>
struct min_op {
    /**
     * @brief 本体
     *
     * @param l
     * @param r
     * @return T
     */
    static T exec(const T l, const T r) { return l < r ? l : r; }
};

/**
 * @brief 2項の最大値求める
 *
 * @tparam T
 */
template <typename T>
struct max_op {
    /**
     * @brief 本体
     *
     * @param l
     * @param r
     * @return T
     */
    static T exec(const T l, const T r) { return l > r ? l : r; }
};

/**
 * @brief テンプレート再帰の末尾
 *
 * @tparam F 二項演算
 * @tparam T
 * @param v
 * @return T
 */
template <typename F, typename T>
inline T multi_op(T&& v) {
    return v;
}

/**
 * @brief 複数項における演算の結果を返す
 *
 * @tparam F
 * @tparam T
 * @tparam Ts
 * @param head
 * @param tail
 * @return T
 */
template <typename F, typename T, typename... Ts>
inline T multi_op(const T head, Ts&&... tail) {
    return F::exec(head, multi_op<F>(tail...));
}

/**
 * @brief 複数項の最小値
 * @see multi_op
 * @tparam T
 * @tparam Ts
 * @param head
 * @param tail
 * @return T
 */
template <typename T, typename... Ts>
inline T multi_min(T&& head, Ts&&... tail) {
    return multi_op<min_op<T>>(head, tail...);
}

/**
 * @brief 複数項の最大値
 * @see multi_op
 * @tparam T
 * @tparam Ts
 * @param head
 * @param tail
 * @return T
 */
template <typename T, typename... Ts>
inline T multi_max(T&& head, Ts&&... tail) {
    return multi_op<max_op<T>>(head, tail...);
}

/**
 * @brief
 * 先頭の値をFで参照する関数に基づいて変更できたらする
 * @tparam F
 * @tparam T
 * @tparam Ts
 * @param target
 * @param candidates
 * @return true
 * @return false
 */
template <typename F, typename T, typename... Ts>
inline bool ch_op(T& target, Ts&&... candidates) {
    const T old = target;
    target      = multi_op<F>(target, candidates...);
    return old != target;
}

/**
 * @brief change min
 * @tparam T 型
 * @param target 変更する値
 * @param candidates
 * @return 更新があればtrue
 */
template <typename T, typename... Ts>
inline bool chmin(T& target, Ts&&... candidates) {
    return ch_op<min_op<T>>(target, candidates...);
}

/**
 * @brief chminのmax版
 * @see chmin
 *  @tparam T 型
 * @param target 変更する値
 * @param candidates
 * @return 更新があればtrue
 */
template <typename T, typename... Ts>
inline bool chmax(T& target, Ts&&... candidates) {
    return ch_op<max_op<T>>(target, candidates...);
}
/* #endregion */
/* #region template/Random.hpp*/
/**
 * @file Random.hpp
 * @author btk
 * @brief 乱数生成系
 * @version 0.1
 * @date 2019-07-13
 * @copyright Copyright (c) 2019
 */


//! 乱数のシード値をプロセスIDとして取得
const pid_t pid = getpid();

/**
 * @brief XorShift32の実装
 */
class XorShift32 {
  private:
    //! XorShiftの現在の値
    unsigned value;

    /**
     * @brief XorShift32のアルゴリズムに基づいて value を更新
     */
    inline void update() {
        value = value ^ (value << 13);
        value = value ^ (value >> 17);
        value = value ^ (value << 5);
    }

    /**
     * @brief 値を更新し，更新前の値を返却
     * @return unsigned 呼び出し時の value を用いる
     */
    inline unsigned get() {
        unsigned v = value;
        update();
        return v;
    }

  public:
    /**
     * @brief [0, 2^bit) の範囲の乱数値を取り出す
     * @tparam デフォルトは31
     * @return int
     */
    template <int bit = 31>
    inline int next_int() {
        return (int)(get() >> (32 - bit));
    }

    /**
     * @brief [-2^bit,2^bit)の範囲の乱数値を取り出す
     * @tparam デフォルトは31
     * @return int
     */
    template <int bit = 31>
    inline int next_signed() {
        unsigned v = get();
        return (int)((v >> (31 - bit)) - (1 << (bit)));
    }

    /**
     * @brief next_int呼び出し時の最大値を取得
     * @tparam 31
     * @return int
     */
    template <int bit = 31>
    inline int range_max() {
        return (int)((1u << bit) - 1);
    };

    /**
     * @brief Construct a new XorShift32 object
     * @param seed
     * @details 初期シードをvalueとするXorShift32のインスタンスを生成
     */
    XorShift32(const unsigned seed) {
        value = seed;
        update();
    }

    /**
     * @brief Construct a new XorShift 32 object
     * @details 初期シードをプロセスIDとするXorShift32のインスタンスを生成
     */
    XorShift32() : XorShift32(pid) {}
};
/* #endregion */
/* #region Template.hpp*/
/**
 * @file Template.hpp
 * @brief 競技プログラミング用テンプレート
 * @author btk15049
 * @date 2019/05/02
 */


/* #endregion */
/* #endregion */
/*</body>*/
#endif


int main() {
    /* write here */
    unordered_set<string> all;
    int n;
    cin >> n;
    string s;
    cin >> s;
    all.insert("");
    for (int i : range(n)) {
        auto nxt = all;
        for (auto& it : all) {
            it += string(1,s[i]);
            if (it.size() <= 3) nxt.insert(it);
        }
        swap(all,nxt);
    }
    int ret = 0;
    for (auto &it : all) {
        if (it.size() == 3) ret++;
    }
    cout << ret << endl;
    return 0;
}


Compiler output:
a.cc: In function 'int main()':
a.cc:516:16: error: no match for 'operator+=' (operand types are 'const std::__cxx11::basic_string<char>' and 'std::string' {aka 'std::__cxx11::basic_string<char>'})
  516 |             it += string(1,s[i]);
      |             ~~~^~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/string:54,
                 from /usr/include/c++/14/bitset:52,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:52,
                 from a.cc:15:
/usr/include/c++/14/bits/basic_string.h:1377:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]' (near match)
 1377 |       operator+=(const basic_string& __str)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1377:7: note:   passing 'const std::__cxx11::basic_string<char>*' as 'this' argument discards qualifiers
/usr/include/c++/14/bits/basic_string.h:1424:9: note: candidate: 'template<class _Tp> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv<_Tp, std::__cxx11::basic_string<_CharT, _Traits, _Alloc>&> std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(const _Tp&) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1424 |         operator+=(const _Tp& __svt)
      |         ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1424:9: note:   template argument deduction/substitution failed:
In file included from /usr/include/c++/14/bits/stl_pair.h:60,
                 from /usr/include/c++/14/bits/stl_algobase.h:64,
                 from /usr/include/c++/14/algorithm:60,
                 from /usr/include/x86_64-linux-gnu/c++/14/bits/stdc++.h:51:
/usr/include/c++/14/type_traits: In substitution of 'template<bool _Cond, class _Tp> using std::enable_if_t = typename std::enable_if::type [with bool _Cond = false; _Tp = std::__cxx11::basic_string<char>&]':
/usr/include/c++/14/bits/basic_string.h:149:8:   required by substitution of 'template<class _CharT, class _Traits, class _Alloc> template<class _Tp, class _Res> using std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::_If_sv = std::enable_if_t<((bool)std::__and_<std::is_convertible<const _Tp&, std::basic_string_view<_CharT, _Traits> >, std::__not_<std::is_convertible<const _Tp*, const std::__cxx11::basic_string<_CharT, _Traits, _Alloc>*> >, std::__not_<std::is_convertible<const _Tp&, const _CharT*> > >::value), _Res> [with _Tp = std::__cxx11::basic_string<char>; _Res = std::__cxx11::basic_string<char>&; _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
  149 |         using _If_sv = enable_if_t<
      |               ^~~~~~
/usr/include/c++/14/bits/basic_string.h:1424:2:   required by substitution of 'template<class _Tp> std::__cxx11::basic_string<char>::_If_sv<_Tp, std::__cxx11::basic_string<char>&> std::__cxx11::basic_string<char>::operator+=(const _Tp&) [with _Tp = std::__cxx11::basic_string<char>]'
 1424 |         operator+=(const _Tp& __svt)
      |         ^~~~~~~~
a.cc:516:32:   required from here
  516 |             it += string(1,s[i]);
      |                                ^
/usr/include/c++/14/type_traits:2711:11: error: no type named 'type' in 'struct std::enable_if<false, std::__cxx11::basic_string<char>&>'
 2711 |     using enable_if_t = typename enable_if<_Cond, _Tp>::type;
      |           ^~~~~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1387:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(const _CharT*) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1387 |       operator+=(const _CharT* __s)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1387:32: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'const char*'
 1387 |       operator+=(const _CharT* __s)
      |                  ~~~~~~~~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:1397:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(_CharT) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1397 |       operator+=(_CharT __c)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1397:25: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'char'
 1397 |       operator+=(_CharT __c)
      |                  ~~~~~~~^~~
/usr/include/c++/14/bits/basic_string.h:1411:7: note: candidate: 'std::__cxx11::basic_string<_CharT, _Traits, _Alloc>& std::__cxx11::basic_string<_CharT, _Traits, _Alloc>::operator+=(std::initializer_list<_Tp>) [with _CharT = char; _Traits = std::char_traits<char>; _Alloc = std::allocator<char>]'
 1411 |       operator+=(initializer_list<_CharT> __l)
      |       ^~~~~~~~
/usr/include/c++/14/bits/basic_string.h:1411:43: note:   no known conversion for argument 1 from 'std::string' {aka 'std::__cxx11::basic_string<char>'} to 'std::initializer_list<char>'
 1411 |       operator+=(initializer_list<_CharT> __l)
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~^~~