洛谷P5854 【模板】笛卡尔树 题解 笛卡尔树模板题

题目链接：https://www.luogu.com.cn/problem/P5854

笛卡尔树 模板题。

示例程序：

2026.9.2

#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int maxn = 1e7 + 5;

int n, a[maxn], ls[maxn], rs[maxn];

void build_tree() {
	stack<int> stk;
	for (int i = 1; i <= n; i++) {
		int last = 0;
		while (!stk.empty() && a[stk.top()] > a[i]) {
			last = stk.top();
			stk.pop();
		}
		if (!stk.empty())
			rs[stk.top()] = i;
		if (last)
			ls[i] = last;
		stk.push(i);
	}
}

int main() {
	scanf("%d", &n);
	for (int i = 1; i <= n; i++)
		scanf("%d", a+i);
	build_tree();
	ll ansl = 0, ansr = 0;
	for (int i = 1; i <= n; i++) {
		ansl ^= 1ll * i * (ls[i] + 1);
		ansr ^= 1ll * i * (rs[i] + 1);
	}
	printf("%lld %lld\n", ansl, ansr);
	return 0;
}

---

#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e7 + 5;

int n;

struct Node {
    int s[2], p, pri;
} tr[maxn];

void f_s(int p, int u, int k) {
    tr[p].s[k] = u;
    tr[u].p = p;
}

void build_tree() {
    stack<int> stk;
    stk.push(0);
    for (int i = 1; i <= n; i++) {
        int pos = stk.top();
        while (!stk.empty() && tr[stk.top()].pri > tr[i].pri) {
            pos = tr[stk.top()].p;
            stk.pop();
        }
        f_s(i, tr[pos].s[1], 0);
        f_s(pos, i, 1);
        stk.push(i);
    }
}

int main() {
    scanf("%d", &n);
    for (int i = 1; i <= n; i++)
        scanf("%d", &tr[i].pri);
    build_tree();
    long long res1 = 0, res2 = 0;
    for (int i = 1; i <= n; i++) {
        res1 ^= 1ll * i * (tr[i].s[0] + 1);
        res2 ^= 1ll * i * (tr[i].s[1] + 1);
    }
    printf("%lld %lld\n", res1, res2);
    return 0;
}