Iterative SegTree

#include <bits/stdc++.h>

using namespace std;

int N, Size = 1;
vector < int > Tree;

void init(int N) {
    while(Size < N)
        Size <<= 1;
    Tree.resize((Size << 1) | 1);
}

int sum(int st, int dr) {
    st += Size, dr += Size;
    int s = 0;
    while(st <= dr) {
        if(st & 1)
            s += Tree[st++];
        if(!(dr & 1))
            s += Tree[dr--];
        st >>= 1;
        dr >>= 1;
    }
    return s;
}

void add(int k, int x) {
    k += Size;
    Tree[k] += x;
    for(k >>= 1; k > 0; k >>= 1)
        Tree[k] = Tree[k << 1] + Tree[(k << 1) | 1];
}

int main() {
    cin >> N;
    init(N);
    for(int i = 1; i <= N; ++i) {
        int x;
        cin >> x;
        add(i, x);
    }
    cout << sum(1, N);
}