线段树只过了一个点求救QAQ

#include <bits/stdc++.h>
#define FAST ios::sync_with_stdio(false); cin.tie(0); cout.tie(0)
#define debug(x) cout<<#x<<": "<<x<<endl
#define ls p << 1
#define rs (p << 1) + 1
typedef long long ll;
using namespace std;
const int N = 5e5 + 50;
const int INF = 1e9;

int n, m, a[N];

struct Segment_Tree {
    int l, r, lsum, rsum, sum, ans;
} tr[N << 2];

void pushup(int p) {
    tr[p].ans = max(max(tr[ls].ans, tr[rs].ans), tr[ls].rsum + tr[rs].lsum);
    tr[p].lsum = max(tr[ls].lsum, tr[ls].sum + tr[rs].lsum);
    tr[p].rsum = max(tr[rs].rsum, tr[ls].rsum + tr[rs].sum);
    tr[p].sum = tr[ls].sum + tr[rs].sum;
}

void build(int p, int l, int r) {
    tr[p].l = l, tr[p].r = r;
    tr[p].ans = tr[p].lsum = tr[p].rsum = tr[p].sum = -INF;

    if(l == r) {
        tr[p].ans = tr[p].lsum = tr[p].rsum = tr[p].sum = a[l];
        return ;
    }

    int mid = (l + r) >> 1;
    build(ls, l, mid);
    build(rs, mid + 1, r);

    pushup(p);
}

void updata(int p, int pos, int num) {
    if(tr[p].l == tr[p].r) {
        tr[p].ans = tr[p].lsum = tr[p].rsum = tr[p].sum = num;
        return ;
    }

    int mid = (tr[p].l + tr[p].r) >> 1;
    if(pos <= mid) updata(ls, pos, num);
    else updata(rs, pos, num);

    pushup(p);
}

int query(int p, int l, int r) {
    if(tr[p].l >= l && tr[p].r <= r) return tr[p].ans;

    int mid = (tr[p].l + tr[p].r) >> 1, ans = -INF;
    if(l <= mid && r >= mid + 1)
        ans = max(max(query(ls, l, r), query(rs, l, r)), tr[ls].rsum + tr[rs].lsum);
    else if(r <= mid) ans = query(ls, l, r);
    else if(l > mid) ans = query(rs, l, r);

    return ans;
}

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

    build(1, 1, n);

    int q, l, r;
    for(int i=1; i<=m; ++i) {
        cin>>q>>l>>r;
        if(q == 1) {
            if(l > r) swap(l, r);
            cout<<query(1, l, r)<<endl;
        } else updata(1, l, r);
    }

    return 0;
}