Data Structures/optimized_segment_tree.cpp


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#include <iostream>
using namespace std;

const int N = 1e5;  // limit for array size
int n;  // array size
int t[2 * N];

void build() {  // build the tree
	for (int i = n - 1; i > 0; --i) t[i] = t[i << 1] + t[i << 1 | 1];
}

void modify(int p, int value) {  // set value at position p
	for (t[p += n] = value; p > 1; p >>= 1) t[p >> 1] = t[p] + t[p ^ 1];
}

int query(int l, int r) {  // sum on interval [l, r)
	int res = 0;
	for (l += n, r += n; l < r; l >>= 1, r >>= 1) {
		if (l & 1) res += t[l++];
		if (r & 1) res += t[--r];
	}
	return res;
}

int main() {
	cin >> n;
	for (int i = 0; i < n; ++i) cin >> t[i];
	build();
	modify(0, 1);
	cout << query(3, 11) << endl;
	return 0;
}