Square Root Decomp

#include<bits/stdc++.h>
using namespace std ;
typedef long long int ll ;
const int N = 1 << 20 ;
ll a[N] , b_index , b_size , lol[N] , n ;
void update(ll pos , ll val ) {
        lol[pos / b_size] = lol[pos / b_size] - a[pos] + val ;
        a[pos] = val ;
}
ll query(ll left , ll right ) {

        ll sum = 0 ;
        while(left <= right and left % b_size != 0 ) {
                sum += a[left] ;
                left++ ;
        }
        while(left + b_size <= right){
               sum += lol[left / b_size] ;
               left += b_size ;
        }
        while(left <= right) {
                sum += a[left] ;
                left++ ;
        }
        return sum ;
}
void add() {
        b_index = -1 ;
        b_size = sqrt(n) ;
        for(int i = 0 ; i < n ;i++ ) {
                if( i  % b_size == 0 )
                        b_index++ ;
                lol[b_index] += a[i] ;
        }
}
int main () {
        cin >> n ;
        for(int  i = 0 ; i < n ; i++) {
                cin >> a[i] ;
        }
        add() ;
        int q ;
        cin >> q ;
        while(q--) {
                int x , y , z ;
                cin >> z ;
                if(z == 1) {
                        int pos , val ;
                        cin >> pos >> val ;
                        update(pos , val ) ;
                }
                else {
                        ll left , right ;
                        cin >> left >> right ;
                        cout << query(left - 1 , right - 1  ) << endl ;
                }

        }

 }