segment_tree.cpp

#include <functional>
#include <iostream>
#include <vector>

using namespace std;

class segment_tree {
  vector<int> tree;
  function<int(int, int)> op;  // Associative operation.

  void build(int v, int tl, int tr, vector<int> a) {
    if (tl == tr) {
      tree[v] = a[tl];
      return;
    }
    int tm = (tl + tr) / 2;
    build(2 * v, tl, tm, a);
    build(2 * v + 1, tm + 1, tr, a);
    tree[v] = op(tree[v * 2], tree[v * 2 + 1]);
  }

 public:
  segment_tree(vector<int> array, function<int(int, int)> op_) {
    op = op_;
    int n = array.size();
    tree.resize(4 * n);
    build(1, 0, n - 1, array);
  }
  // v - vertex to start from, default is 0
  // tl - left bound of tree to search from
  // tr - right bound of tree to search to
  // ql - left bound of query
  // qr - right bound of query
  int query(int tl, int tr, int ql, int qr, int v = 1) {
    if (ql > qr) {
      return 0;
    }
    if (tl == ql && tr == qr) {
      return tree[v];
    }
    int tm = (tl + tr) / 2;
    int left_res = query(tl, tm, ql, min(qr, tm), v * 2);
    int right_res = query(tm + 1, tr, max(ql, tm + 1), qr, v * 2 + 1);
    return op(left_res, right_res);
  }
};
int main() {
  int n, x;
  cin >> n;
  vector<int> initial_array;
  for (int i = 0; i < n; i++) {
    cin >> x;
    initial_array.push_back(x);
  }

  segment_tree tree(initial_array, [](int a, int b) { return a + b; });
  int l, r;
  cin >> l >> r;
  cout << tree.query(0, n - 1, l, r) << endl;
  return 0;
}