从零开始学算法/C++/Start

今天学会了线段树，线段树实在是太难了，不过很好用
[https://www.acwing.com/problem/content/246/](AcWing 245 你能回答这些问题吗)
明天尝试学到算法竞赛进阶指南p245，然后再把洛谷题单里面线段树题单刷完，说实话很有难度。
洛谷的题目和数据太有水平了。
下面是该题代码

#include <iostream>
#include <algorithm>
using namespace std;
const int N = 5e5 + 1;
int w[N];
struct node
{
	int l, r, lmax, rmax, sum, data;
}t[N << 2];
void build(int p, int l, int r)
{
	t[p] = { l,r };
	if (l == r) { t[p] = { l,l,w[l],w[l],w[l],w[l] }; return; }
	int mid = l + r >> 1;
	build(p << 1, l, mid), build(p << 1 | 1, mid + 1, r);
	t[p].sum = t[p << 1].sum + t[p << 1 | 1].sum;
	t[p].lmax = max(t[p << 1].lmax, t[p << 1].sum + t[p << 1 | 1].lmax);
	t[p].rmax = max(t[p << 1 | 1].rmax, t[p << 1 | 1].sum + t[p << 1].rmax);
	t[p].data = max(t[p << 1].rmax + t[p << 1 | 1].lmax, max(t[p << 1].data, t[p << 1 | 1].data));
}
node ffind(int p, int l, int r)
{
	if (l <= t[p].l && t[p].r <= r)return t[p];
	int mid = t[p].l + t[p].r >> 1;
	if (r <= mid)return ffind(p << 1, l, r);
	else if (l > mid)return ffind(p << 1 | 1, l, r);
	else
	{
		node a, b, c;
		a = ffind(p << 1, l, r);
		b = ffind(p << 1 | 1, l, r);
		c.sum = a.sum + b.sum;
		c.lmax = max(a.lmax, a.sum + b.lmax);
		c.rmax = max(b.rmax, b.sum + a.rmax);
		c.data = max(a.rmax + b.lmax, max(a.data, b.data));
		return c;
	}
}
void modify(int p, int x, int k)
{
	if (t[p].l == t[p].r) { t[p] = { t[p].l,t[p].r,k,k,k,k }; return; }
	int mid = t[p].l + t[p].r >> 1;
	if (x <= mid)modify(p << 1, x, k);
	else modify(p << 1 | 1, x, k);
	t[p].sum = t[p << 1].sum + t[p << 1 | 1].sum;
	t[p].lmax = max(t[p << 1].lmax, t[p << 1].sum + t[p << 1 | 1].lmax);
	t[p].rmax = max(t[p << 1 | 1].rmax, t[p << 1 | 1].sum + t[p << 1].rmax);
	t[p].data = max(t[p << 1].rmax + t[p << 1 | 1].lmax, max(t[p << 1].data, t[p << 1 | 1].data));
}
int n, m, k, x, y;
int main()
{
	ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
	cin >> n >> m;
	for (int i = 1; i <= n; ++i)cin >> w[i];
	build(1, 1, n);
	while (m--)
	{
		cin >> k >> x >> y;
		if (k == 1)
		{
			if (x > y)swap(x, y);
			cout << ffind(1, x, y).data << endl;
		}
		else modify(1, x, y);
	}
	return 0;
}