Author: lllyouo
Date: 20251014
tag: 线段树
link: https://www.acwing.com/problem/content/246/

问题描述

link

分析

略

参考代码

#include <bits/stdc++.h>
using namespace std;

const int N = 5e5 + 10;

struct SegmentTree {
    int l, r;
    int tmax, lmax, rmax, sum;
} t[4 * N];
int a[N], n, m;

void pushup(SegmentTree& p, SegmentTree& l, SegmentTree& r) {
    p.sum = l.sum + r.sum;
    p.lmax = max(l.lmax, l.sum + r.lmax);
    p.rmax = max(r.rmax, r.sum + l.rmax);
    p.tmax = max(max(l.tmax, r.tmax), l.rmax + r.lmax);
}

void pushup(int p) {
    pushup(t[p], t[p << 1], t[p << 1 | 1]);
}

void build(int p, int l, int r) {
    t[p].l = l, t[p].r = r;
    if (l == r) {
        t[p].tmax = t[p].sum = t[p].lmax = t[p].rmax = a[l];
        return;
    }
    int mid = (l + r) >> 1;
    build(p << 1, l, mid);
    build(p << 1 | 1, mid + 1, r);
    pushup(p);
}

void modify(int p, int x, int v) {
    if (t[p].l == x && t[p].r == x) {
        t[p].tmax = t[p].sum = t[p].lmax = t[p].rmax = v;
        return;
    }
    int mid = (t[p].l + t[p].r) >> 1;
    if (x <= mid) modify(p << 1, x, v);
    else modify(p << 1 | 1, x, v);
    pushup(p);
}

SegmentTree query(int p, int l, int r) {
    if (t[p].l >= l && t[p].r <= r) return t[p];
    int mid = (t[p].l + t[p].r) >> 1;
    // 注意
    if (r <= mid) return query(p << 1, l, r);
    if (l > mid) return query(p << 1 | 1, l, r);
    else {
        auto left = query(p << 1, l, r);
        auto right = query(p << 1 | 1, l, r);
        SegmentTree temp;
        pushup(temp, left, right);
        return temp;
    }
}

int main() {
    cin >> n >> m;
    for (int i = 1; i <= n; i++) cin >> a[i];
    build(1, 1, n);

    while (m --) {
        int k, x, y; cin >> k >> x >> y;
        if (k == 1) {
            if (x > y) swap(x, y);
            cout << query(1, x, y).tmax << endl;
        } else {
            modify(1, x, y);
        }
    }

    return 0;
}