Luogu P4513 小白逛公园

题目链接：​​传送门​​

区间最大子段和
就因为两个挨着的小错误困扰了我十几个小时…
好气

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cstdlib>
#include <complex>
#include <algorithm>
#include <climits>
#include <queue>
#include <map>
#include <set>
#include <vector>
#include <iomanip>
#define
#define

using namespace std;
typedef long long ll;
struct node {
    int l, r, s, ls, rs, sz;
}tree[A];
int n, m, opt, a, b;
void up(int k) {
    tree[k].sz = tree[k << 1].sz + tree[k << 1 | 1].sz;
    tree[k].s = max(tree[k << 1].s, tree[k << 1 | 1].s);
    tree[k].s = max(tree[k].s, tree[k << 1].rs + tree[k << 1 | 1].ls);
    tree[k].ls = max(tree[k << 1].ls, tree[k << 1].sz + tree[k << 1 | 1].ls);
    tree[k].rs = max(tree[k << 1 | 1].rs, tree[k << 1 | 1].sz + tree[k << 1].rs);
}
void build(int k, int l, int r) {
    tree[k].l = l; tree[k].r = r;
    if (l == r) {
        scanf("%d", &tree[k].sz);
        tree[k].s = tree[k].ls = tree[k].rs = tree[k].sz;
        return;
    }
    int m = (l + r) >> 1;
    build(k << 1, l, m);
    build(k << 1 | 1, m + 1, r);
    up(k);
}
void change(int k, int pos, int val) {
    if (tree[k].l == tree[k].r) {
        tree[k].sz = tree[k].s = tree[k].ls = tree[k].rs = val;
        return;
    }
    int m = (tree[k].l + tree[k].r) >> 1;
    if (pos <= m) change(k << 1, pos, val);
    else change(k << 1 | 1, pos, val);
    up(k);
}
node ask(int k, int l, int r) {
    if (tree[k].l >= l and tree[k].r <= r) return tree[k];
    int m = (tree[k].l + tree[k].r) >> 1;
    if (m >= l and m < r) {
        node lso = ask(k << 1, l, r), rso = ask(k << 1 | 1, l, r), res;
        res.s = max(lso.s, rso.s);
        res.s = max(res.s, lso.rs + rso.ls);
        res.ls = max(lso.ls, lso.sz + rso.ls);
        res.rs = max(rso.rs, rso.sz + lso.rs);
        return res;
    }
    if (l <= m) return ask(k << 1, l, r);
    if (r > m) return ask(k << 1 | 1, l, r);
}

int main(int argc, char const *argv[]) {
    cin >> n >> m; build(1, 1, n);
    while (m--) {
        scanf("%d%d%d", &opt, &a, &b);
        if (opt == 1) {
            if (a > b) swap(a, b);
            printf("%d\n", ask(1, a, b).s);
        }
        else change(1, a, b);
    }
    return 0;
}