luoguP3521 [POI2011]ROT-Tree Rotations【线段树】

# include "bits/stdc++.h"
using namespace std;
int main() {
	int n;
	cin >> n;
	long long answer_without_exchange, answer_after_exchange;
	struct node {
		int l, r, siz;
	};
	# define ls t[rt].l
	# define rs t[rt].r
	# define lson t[rt].l, l, mid
	# define rson t[rt].r, mid + 1, r
	vector<node> t((n + 1) * (log(n + 1) + 1) * 2); // seriously, there should * 2
	auto push_up = [&](int rt) -> void {
		t[rt].siz = t[ls].siz + t[rs].siz;
	};
	int tree_index = 0;
	vector<int> rt(n + 1);
	auto update = [&](auto update, int rt, int l, int r, int x) -> int {
		t[rt].siz = 1; // first set the size of all nodes to 1, ensuring the validity of the multiplication operation in the subsequent anwer statistics
		if(l == r) {
//			t[rt].siz = 1;
			return rt;
		}
		int mid = l + r >> 1;
		if(x <= mid)
			t[rt].l = update(update, ++tree_index, l, mid, x);
		else
			t[rt].r = update(update, ++tree_index, mid + 1, r, x);
		return rt;
	};
	auto merge = [&](auto merge, int x, int y, int l, int r) -> int {
		if(!x || !y) return x | y;
		if(l == r) {
			t[x].siz += t[y].siz;
			return x;
		}
		answer_without_exchange += 1ll * t[t[x].l].siz * t[t[y].r].siz;
		answer_after_exchange += 1ll * t[t[x].r].siz * t[t[y].l].siz;
		int mid = l + r >> 1;
		t[x].l = merge(merge, t[x].l, t[y].l, l, mid);
		t[x].r = merge(merge, t[x].r, t[y].r, mid + 1, r);
		push_up(x);
		return x;
	};
	long long ans = 0;
	auto dfs = [&](auto dfs) -> int {
		int now, val;
		cin >> val;
		if(val == 0) {
			int left_subtree = dfs(dfs), right_subtree = dfs(dfs); // preorder traversal
			answer_without_exchange = 0;
			answer_after_exchange = 0;
			now = merge(merge, left_subtree, right_subtree, 1, n);
			ans += min(answer_without_exchange, answer_after_exchange);
		}
		else {
			now = update(update, ++tree_index, 1, n, val);
		}
		return now;
	};
	dfs(dfs);
	cout << ans;
	return 0;
}