Language: C++

#include <iostream>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <algorithm>
#include <climits>
#define ll long long
#define MAX 25 * 1e13
#define N  200010

using namespace std;
int MOD = 1000000007;


#define MAX_Q 1001000

using namespace std;
int max_int = 1e9;

int dat[MAX_Q] = {0};
int n;
int init( int n_ ){
        n = 1;
        while( n < n_ ) n *= 2;
        return 0;
}
int add( int k , char x){// update with a point
        // [a,b) is target interval
        // x is the value added on interval
        // k is node id
        // [l,r) is current interval
        k += n-1;//first n-2 node is not leaf,last n nodes are leaf,adjust target position
        dat[k] = 0;
        dat[k] |= (1 << (x -'a')); // update the very point
        while( k > 0 ){// update to the root
                k = (k - 1 ) /2;//find the father id
                dat[k] = dat[k*2+1] | dat[k*2+2];//update the sum of father node
        }
        return 0;
}
int query_sum( int a , int b , int k , int l , int r ){// query sum of value with a span
        if( b <= l || r <= a ){// no overlap part
                return 0;
        }
        else if( a <= l && r <= b ){// wholely overlap
                return dat[k];//current node value
        }
        else{//partly overlap [l,r)
                int res = 0;
                res |= query_sum( a , b , k * 2 + 1 , l , (l+r)/2 );//search left son 
                res |= query_sum( a , b , k * 2 + 2 , (l+r)/2 , r );//search right son

                return res;

        }

}
int print(){
        cout << "Array  : ";
        int pos = n-1;//offset
        for( int i = 0 ; i < n ; i++ ){
                cout << dat[i + pos] << " ";
        }
        cout << endl;
        return 0;
}

string str;
int q;

int main(){

        cin.tie(0);
        ios::sync_with_stdio(false);

        int n_;

        cin >> n_;
        cin >> str;
        cin >> q;

        memset(dat,0,sizeof(dat));//initial all the data in the inerval tree
        int tmp;
        init(n_);
        for( int j = 0 ; j < n_ ; j++ ){
                add( j, str[j] );// span: [ 0 , n )
        }
        char op1[100];
        int t = 0;
        //print();
        while( t < q ){
                t++;
                int op;
                cin >> op ;
                if( op == 1 ){
                        
                        int pt;// update by point
                        
                        string val;
                        cin >> pt >> val;
                        add( pt-1 ,val[0] );
                        //print();
                        continue;
                }
                if( op == 2  ){
                        int l,r;
                        cin >> l >> r;
                        int ret = query_sum( l-1, r , 0 , 0 , n );
                        int cnt = 0;
                        for(int i = 0 ; i < 26 ; i++ ){
                          if( (ret >> i) & 1   ) cnt++;
                        }
                        cout << cnt << endl;
                        continue;
                }
        }//end for while op
        return 0;
}//end main



Compiler output:
a.cc: In function 'int main()':
a.cc:83:9: error: 'memset' was not declared in this scope
   83 |         memset(dat,0,sizeof(dat));//initial all the data in the inerval tree
      |         ^~~~~~
a.cc:8:1: note: 'memset' is defined in header '<cstring>'; this is probably fixable by adding '#include <cstring>'
    7 | #include <climits>
  +++ |+#include <cstring>
    8 | #define ll long long