题目链接（搬运自AcWing245）

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

#define int long long
#define INF 1e15 
#define maxn 500005
struct SegmentTree {
    int l, r;
    int sum, lmax, rmax, dat;
} t[maxn];
int n, m;
int a[maxn];

void build(int p, int l, int r) {
    t[p].l = l, t[p].r = r;
    if (l == r) { t[p].dat = t[p].sum = t[p].lmax = t[p].rmax = t[p].dat = a[l]; return; }
    int mid = (l + r) >> 1;
    build(p * 2, l, mid), build(p * 2 + 1, mid + 1, r);
    t[p].sum = t[p * 2].sum + t[p * 2 + 1].sum;
    t[p].lmax = max(t[p * 2].lmax, t[p * 2 + 1].lmax + t[p * 2].sum);
    t[p].rmax = max(t[p * 2].rmax + t[p * 2 + 1].sum, t[p * 2 + 1].sum);
    t[p].dat = max(max(t[p * 2].dat, t[p * 2 + 1].dat), t[p * 2].rmax + t[p * 2 + 1].lmax);
}

int ask(int p, int l, int r) {
    if (t[p].l == t[p].r) return t[p].dat;
    int mid = (t[p].l + t[p].r) >> 1, val = -INF;
    if (l <= mid) val = max(val, ask(p * 2, l, mid));
    if (r > mid) val = max(val, ask(p * 2 + 1, mid + 1, r));
    return val;
}

void change(int p, int x, int v) {
    if (t[p].l == t[p].r) {
        t[p].sum = t[p].lmax = t[p].rmax = t[p].dat = v;
        return;
    }
    int mid = (t[p].l + t[p].r) >> 1;
    if (x <= mid) change(p * 2, x, v);
    else change(p * 2 + 1, x, v);
    t[p].sum = t[p * 2].sum + t[p * 2 + 1].sum;
    t[p].lmax = max(t[p * 2].lmax, t[p * 2 + 1].lmax + t[p * 2].sum);
    t[p].rmax = max(t[p * 2].rmax + t[p * 2 + 1].sum, t[p * 2 + 1].sum);
    t[p].dat = max(max(t[p * 2].dat, t[p * 2 + 1].dat), t[p * 2].rmax + t[p * 2 + 1].lmax);
}

signed main() {
    cin >> n >> m;
    for (int i = 1; i <= n; i++) cin >> a[i];
    build(1, 1, n);
    for (int i = 1; i <= m; i++) {
        int k, x, y;
        cin >> k >> x >> y;
        if (x > y) swap(x, y);
        if (k == 1) cout << ask(1, x, y) << endl;
        else change(1, x, y);
    }
    return 0;
}

看了题解感觉是ask函数出了问题，题解里的ask函数里还写了其他东西，不过感觉不是很懂，有没有大佬解释一下？