Language: C++

#include <cstdio>
#include <cstring>
#include <cmath>
#include <utility>
#include <iostream>
#include <bitset>
#include <algorithm>
#include <vector>
#include <forward_list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#define rep(i, s, g) for ((i) = (s); (i) < (g); ++(i))
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
const ll MOD = 1e9 + 7;
const ll INF = (1ll << 60);

template <typename T>
class segmenttree
{
private:
    int n;

public:
    using F = function<T(T, T)>;

    T dif;
    F f;
    vector<T> st;

    segmenttree(int size, T init, F f) : f(f)
    {
        n = 1;

        while (n < size)
        {
            n *= 2;
        }

        st.assign(2 * n - 1, init);
    }

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

    void build()
    {
        for (int i = n - 2; i >= 0; i--)
        {
            st[i] = f(st[i * 2 + 1], st[i * 2 + 2]);
        }
    }

    void update(int i, T x)
    {
        i += n - 1;
        st[i] = x;

        while (i > 0)
        {
            i = (i - 1) / 2;
            st[i] = f(st[2 * i + 1], st[2 * i + 2]);
        }
    }

    T query_(int a, int b, int k, int l, int r)
    {
        if (r <= a || b <= l)
        {
            return dif;
        }

        if (a <= l && r <= b)
        {
            return st[k];
        }
        else
        {
            T x = query_(a, b, 2 * k + 1, l, (l + r) / 2);
            T y = query_(a, b, 2 * k + 2, (l + r) / 2, r);

            return f(x, y);
        }
    }

    T query(int a, int b)
    {
        return query_(a, b, 0, 0, n);
    }
};

// int n;
// vector<bitset<26>> seg;
// string s;

// void init(int n_)
// {
//     n = 1;

//     while (n < n_)
//     {
//         n *= 2;
//     }
// }

// void update(int i, char x)
// {
//     seg[i + n - 1].reset(s[i] - 'a');
//     s[i] = x;
//     seg[i + n - 1].set(s[i] - 'a');

//     i += n - 1;

//     while (i > 0)
//     {
//         i = (i - 1) / 2;
//         seg[i] = seg[2 * i + 1] | seg[2 * i + 2];
//     }
// }

// bitset<26> query(int a, int b, int k, int l, int r)
// {
//     if (r <= a || b <= l)
//     {
//         bitset<26> bit(0ll);
//         return bit;
//     }

//     if (a <= l && r <= b)
//     {
//         return seg[k];
//     }
//     else
//     {
//         bitset<26> bit1 = query(a, b, 2 * k + 1, l, (l + r) / 2);
//         bitset<26> bit2 = query(a, b, 2 * k + 2, (l + r) / 2, r);

//         return bit1 | bit2;
//     }
// }

int main(void)
{
    int n, q;
    string s;
    int i, j;
    vector<int> ans;

    cin >> n >> s >> q;

    segmenttree<bitset<26>> seg(n, 0, [](bitset<26> a, bitset<26> b) { return a | b; });
    seg.dif = 0;

    rep(i, 0, n)
    {
        bitset<26> bit = 1 << (s[i] - 'a');
        seg.set(i, bit);
    }

    seg.build();

    rep(i, 0, q)
    {
        int x;

        cin >> x;

        if (x == 1)
        {
            int iq;
            char c;
            bitset<26> bit;

            cin >> iq >> c;
            iq--;

            s[iq] = c;
            bit = 1 << (s[iq] - 'a');

            seg.update(iq, bit);
        }
        else
        {
            int l, r;
            int an = 0;

            cin >> l >> r;
            l--;

            bitset<26> bit = seg.query(l, r);

            an = bit.count();

            ans.push_back(an);
        }
    }

    for (auto &z : ans)
    {
        cout << z << endl;
    }
}

Compiler output:
a.cc:28:15: error: 'function' does not name a type; did you mean 'union'?
   28 |     using F = function<T(T, T)>;
      |               ^~~~~~~~
      |               union
a.cc:31:5: error: 'F' does not name a type
   31 |     F f;
      |     ^
a.cc:34:35: error: 'F' has not been declared
   34 |     segmenttree(int size, T init, F f) : f(f)
      |                                   ^
a.cc: In constructor 'segmenttree<T>::segmenttree(int, T, int)':
a.cc:34:42: error: class 'segmenttree<T>' does not have any field named 'f'
   34 |     segmenttree(int size, T init, F f) : f(f)
      |                                          ^
a.cc: In function 'int main()':
a.cc:156:39: error: invalid user-defined conversion from 'main()::<lambda(std::bitset<26>, std::bitset<26>)>' to 'int' [-fpermissive]
  156 |     segmenttree<bitset<26>> seg(n, 0, [](bitset<26> a, bitset<26> b) { return a | b; });
      |                                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc:156:39: note: candidate is: 'constexpr main()::<lambda(std::bitset<26>, std::bitset<26>)>::operator std::bitset<26> (*)(std::bitset<26>, std::bitset<26>)() const' (near match)
  156 |     segmenttree<bitset<26>> seg(n, 0, [](bitset<26> a, bitset<26> b) { return a | b; });
      |                                       ^
a.cc:156:39: note:   no known conversion from 'std::bitset<26> (*)(std::bitset<26>, std::bitset<26>)' to 'int'
a.cc:34:37: note:   initializing argument 3 of 'segmenttree<T>::segmenttree(int, T, int) [with T = std::bitset<26>]'
   34 |     segmenttree(int size, T init, F f) : f(f)
      |                                   ~~^
a.cc: In instantiation of 'void segmenttree<T>::build() [with T = std::bitset<26>]':
a.cc:165:14:   required from here
  165 |     seg.build();
      |     ~~~~~~~~~^~
a.cc:55:22: error: 'f' was not declared in this scope
   55 |             st[i] = f(st[i * 2 + 1], st[i * 2 + 2]);
      |                     ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc: In instantiation of 'void segmenttree<T>::update(int, T) [with T = std::bitset<26>]':
a.cc:185:23:   required from here
  185 |             seg.update(iq, bit);
      |             ~~~~~~~~~~^~~~~~~~~
a.cc:67:22: error: 'f' was not declared in this scope
   67 |             st[i] = f(st[2 * i + 1], st[2 * i + 2]);
      |                     ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
a.cc: In instantiation of 'T segmenttree<T>::query_(int, int, int, int, int) [with T = std::bitset<26>]':
a.cc:93:16:   required from 'T segmenttree<T>::query(int, int) [with T = std::bitset<26>]'
   93 |         return query_(a, b, 0, 0, n);
      |                ^~~~~~
a.cc:195:39:   required from here
  195 |             bitset<26> bit = seg.query(l, r);
      |                              ~~~~~~~~~^~~~~~
a.cc:87:21: error: 'f' was not declared in this scope
   87 |             return f(x, y);
      |                    ~^~~~~~