Binary Indexed Tree (Fenwick Tree) C++ implementation for range update, point query

BIT.cpp

// https://cses.fi/problemset/task/1651/
#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

struct BIT{
    //1-indexed
    vector<ll> bit;
    ll n;
    BIT(ll N){
        n = N;
        bit = vector<ll>(2*n + 1);
    }
    ll get(ll i) { //get sum with prefix sum array, get single value with difference array
        ll ans = 0;
        while (i > 0) {
            ans += bit[i];
            i -= (i & (-i));
        }
        return ans;
    }
    void updateSingle(ll i, ll val) {
        while (i <= n) {
            bit[i] += val; // change += to = or -= to change update mode
            i += (i & (-i));
        }
    }

    void updateRange(ll l, ll r, ll val) {
        updateSingle(l, val);
        updateSingle(r + 1, -val);
    }
};

int main(){
    ll n, q;
    cin >> n >> q;
    BIT a(n);
    ll diff = 0;
    for(ll i = 1; i <= n; i++){
        ll x;
        cin >> x;
        a.updateSingle(i, x - diff);
        diff = x;
    }
    while(q--){
        ll type;
        cin >> type;
        if (type == 1) {
            ll l, r, v;
            cin >> l >> r >> v;
            a.updateRange(l, r, v);
        } else if (type == 2) {
            ll i;
            cin >> i;
            cout << a.get(i) << '\n';
        }
    }
}