Segment Tree C++ iterative implementation (only support point update)

segment_tree_iterative.cpp

#include <bits/stdc++.h>

using namespace std;
typedef long long ll;

struct segment_tree {
    //0-indexed
    vector<ll> tree;
    ll n;
    segment_tree(vector<ll> &nums) {
        n = nums.size();
        tree = vector<ll>(2 * n);
        for (ll i = n; i < 2 * n; i++) {
            tree[i] = nums[i - n];
        }
        for (ll i = n - 1; i > 0; i--) {
            tree[i] = tree[2 * i] + tree[2 * i + 1];
        }
    }
    void update(ll pos, ll val) {
        pos += n;
        tree[pos] += val; // edit "=" to change edit mode
        while (pos > 0) {
            ll l = pos;
            ll r = pos;
            if (pos % 2 == 0)
                r = pos + 1;
            else
                l = pos - 1;
            tree[pos / 2] = tree[l] + tree[r];
            pos /= 2;
        }
    }
    ll query(ll l, ll r) {
        l += n;
        r += n;
        ll ans = 0;
        while (l <= r) {
            if (l % 2 == 1) {
                ans = ans + tree[l];
                l++;
            }
            if (r % 2 == 0) {
                ans = ans + tree[r];
                r--;
            }
            l /= 2;
            r /= 2;
        }
        return ans;
    }
};

int main() {
    vector<ll> v = {1, 3, 2, 4};
    segment_tree tree(v);
    cout << tree.query(0, 3) << '\n';  // 4
    tree.update(0, 5);
    cout << tree.query(0, 3) << '\n';  // 5
}