Simple segment tree (good)

#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef vector<int> vi;



const int MAXN = 300005;

int n;
int arr[MAXN];

int st[4*MAXN];

void build(int v, int tl, int tr) {
    if (tl == tr) {
        st[v] = arr[tl];
    } else {
        int tm = (tl + tr) / 2;
        build(2*v, tl, tm);
        build(2*v+1, tm+1, tr);
        st[v] = st[2*v] + st[2*v+1];
    }
}

int sum(int v, int tl, int tr, int l, int r) {
    if (l > r || l > tr || r < tl) return 0;
    if (l <= tl && tr <= r) return st[v];
    int tm = (tl + tr) / 2;
    return sum(2*v, tl, tm, l, r) + sum(2*v+1, tm+1, tr, l, r);
}

void update(int v, int tl, int tr, int pos, int val) {
    if (tl == tr) {
        st[v] = val;
    } else {
        int tm = (tl + tr) / 2;
        if (pos <= tm) {
            update(2*v, tl, tm, pos, val);
        } else {
            update(2*v+1, tm+1, tr, pos, val);
        }
        st[v] = st[2*v] + st[2*v+1];
    }
}

void Main() {
    cout << "Enter N, the size of the array, and then N numbers\n";
    cin >> n;
    for (int i = 0; i < n; ++i) {
        cin >> arr[i];
    }
    build(1, 0, n-1);
    cout << "First type of query:\n";
    cout << "1 l r\n";
    cout << "--> Get the sum from l to r (0-indexed)\n";
    cout << "Second type of query:\n";
    cout << "2 x v\n";
    cout << "--> Update element at position x to new value v\n";
    while (true) {
        int t;
        cin >> t;
        if (t == 1) {
            int l, r;
            cin >> l >> r;
            cout << "Sum(" << l << ", " << r << ") = " << sum(1, 0, n-1, l, r) << "\n";
        } else if (t == 2) {
            int x, v;
            cin >> x >> v;
            update(1, 0, n-1, x, v);
        } else {
            cout << "Could not understand query\n";
        }
    }
}

int main() {
//    ios::sync_with_stdio(false);
//    cin.tie(0);
    #ifdef _DEBUG
//    freopen("in.txt", "r", stdin);
//    freopen("out.txt", "w", stdout);
    #endif
    #ifndef _DEBUG
    #endif
    Main();
}