dynamic segment tree implementation in c++

dynamic_seg.cpp

#include <bits/stdc++.h>

using namespace std;

// identity element of binary function
int e = 0;

int func(int a, int b) {
	// binary function
	// ex) return gcd(a, b);
	// ex) return a * b;
	// ex) return a ^ b;
	// BUT! associated law has to be held
	return a + b;
}

struct seg_node {
	seg_node *left, *right;
	int value;
};

void init(vector<int> &vec, seg_node *node, int left, int right) {
	if(left == right) {
		node->value = vec[left - 1];
		return;
	}
	node->left = new seg_node;
	node->right = new seg_node;
	init(vec, node->left, left, (left + right) >> 1);
	init(vec, node->right, ((left + right) >> 1) + 1, right);
	node->value = func(node->left->value, node->right->value);
}

void update(seg_node *node, int left, int right, int index, int value) {
	if(index < left || right < index) return;
	if(left == right) {
		node->value = value;
		return;
	}
	if(node->left == nullptr) node->left = new seg_node;
	if(node->right == nullptr) node->right = new seg_node;
	update(node->left, left, (left + right) >> 1, index, value);
	update(node->right, ((left + right) >> 1) + 1, right, index, value);
	node->value = func(node->left->value, node->right->value);
}

int query(seg_node *node, int left, int right, int start, int end) {
	if(right < start || end < left || node == nullptr) return e;
	if(start <= left && right <= end) return node->value;
	int mid = (left + right) >> 1;
	return func(query(node->left, left, mid, start, end), query(node->right, mid + 1, right, start, end));
}

int main() {
	int size = 4;
	auto *root = new seg_node;
	vector<int> v{1, 2, 3, 4};
	init(v, root, 1, size);
	update(root, 1, size, 2, 4);
	cout << query(root, 1, size, 2, 4) << endl;
}