线段树

区间信息维护

#include <bits/stdc++.h>
#define lowbit(x) (x & (-x))
typedef long long ll;
using namespace std;
const int N = 5e5 + 1, inf = 0x3f3f3f3f;
int n, m, a[N];
struct Node {
    int l, r, sum, lmax, rmax, tmax;
} tr[N << 2];

void pushup(Node& u, Node& l, Node& r) {
    u.sum = l.sum + r.sum;
    u.lmax = max(l.lmax, l.sum + r.lmax);
    u.rmax = max(r.rmax, r.sum + l.rmax);
    u.tmax = max(max(l.tmax, r.tmax), l.rmax + r.lmax);
}
void pushup(int u) {
    pushup(tr[u], tr[u << 1], tr[u << 1 | 1]);
}
void build(int u, int l, int r) {
    if (l == r) {
        tr[u] = {l, r, a[l], a[l], a[l], a[l]}; return;
    }
    int mid = l + r >> 1;
    tr[u] = {l, r};
    build(u << 1, l, mid), build(u << 1 | 1, mid + 1, r);
    pushup(u);
}
Node query(int u, int l, int r) {
    if (tr[u].l >= l && tr[u].r <= r) return tr[u];
    int mid = tr[u].l + tr[u].r >> 1;
    if (r <= mid) return query(u << 1, l, r);
    else if (l > mid) return query(u << 1 | 1, l, r);
    else {
        auto left = query(u << 1, l, r);
        auto right = query(u << 1 | 1, l, r);
        Node res; pushup(res, left, right);
        return res;
    }
}
void modify(int u, int x, int v) {
    if (tr[u].l == x && tr[u].r == x) {
        tr[u] = {x, x, v, v, v, v}; return;
    }
    int mid = tr[u].l + tr[u].r >> 1;
    if (x <= mid) modify(u << 1, x, v);
    else modify(u << 1 | 1, x, v);
    pushup(u);
}
int main() {
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= n; i++) scanf("%d", &a[i]);
    build(1, 1, n); int k, x, y;
    while (m--) {
        scanf("%d%d%d", &k, &x, &y);
        if (k == 1) {
            if (x > y) swap(x, y);
            printf("%d\n", query(1, x, y).tmax);
        } else modify(1, x, y);
    }
    return 0;
}