AcWing 245 你能回答这些问题吗？ (线段树）

题目链接：https://www.acwing.com/problem/content/description/246/

经典题，动态维护最大字段和
线段树节点内除了 (sum) 外，还要维护左起最大子段和，右起最大子段和，最大子段和

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<cmath>
#include<stack>
#include<queue>
using namespace std;
typedef long long ll;

const int maxn = 500010;
const int INF = 1000000007;

int n, m;
int a[maxn];

struct SEG{
	int sum, lsum, rsum, msum;
	
	SEG (int sum = 0, int lsum = -INF, int rsum = -INF, int msum = -INF): sum(sum), lsum(lsum), rsum(rsum), msum(msum){}
}t[maxn << 2];

void pushup(int i){
	t[i].sum = t[i << 1].sum + t[i << 1 | 1].sum;
	t[i].lsum = max(t[i << 1].lsum, t[i << 1].sum + t[i << 1 | 1].lsum);
	t[i].rsum = max(t[i << 1 | 1].rsum, t[i << 1].rsum + t[i << 1 | 1].sum);
	t[i].msum = max(max(t[i << 1].msum, t[i << 1 | 1].msum),t[i << 1].rsum + t[i << 1 | 1].lsum);
}

void build(int i, int l, int r){
	if(l == r){
		t[i].sum = t[i].msum = t[i].lsum = t[i].rsum = a[l];
		return;
	}
	int mid = (l + r) >> 1;
	build(i << 1, l, mid); build(i << 1|1, mid + 1, r);
	pushup(i); 
}

void modify(int i, int l, int r, int k, int p){
	if(l == r){
		t[i].lsum = t[i].rsum = t[i].msum = t[i].sum = k;
		return;
	}
	int mid = (l + r) >> 1;
	if(p <= mid) modify(i << 1, l, mid, k, p);
	else modify(i << 1 | 1, mid + 1, r, k, p);
	pushup(i); 
} 

SEG query(int i, int l, int r, int x, int y){
	if(x <= l && r <= y){ 
		return t[i]; 
	} 
	
	int mid = (l + r) >> 1;
	SEG lt, rt, at; 
	if(x <= mid) lt = query(i << 1, l, mid, x, y);
	if(y > mid) rt = query(i << 1 | 1, mid + 1, r, x, y);
	int ls = max(lt.lsum, lt.sum + rt.lsum);
	int rs = max(rt.rsum, rt.sum + lt.rsum);
	int ms = max(max(lt.msum, rt.msum), lt.rsum + rt.lsum);
	int s = lt.sum + rt.sum;
	return SEG(s, ls, rs, ms);
}

ll read(){ ll s=0,f=1; char ch=getchar(); while(ch<'0' || ch>'9'){ if(ch=='-') f=-1; ch=getchar(); } while(ch>='0' && ch<='9'){ s=s*10+ch-'0'; ch=getchar(); } return s*f; }

int main(){
	n = read(), m = read();
	for(int i = 1; i <= n; ++i) a[i] = read();
	
	build(1, 1, n);
	
	int op, x, y;
	for(int i = 1; i <= m; ++i){
		op = read(), x = read(), y = read();
		if(op == 1){
			if(x > y) swap(x, y);
			SEG res = query(1, 1, n, x, y);
			printf("%d
", res.msum);
		}else{
			modify(1, 1, n, y, x);
		}
	}
	
	return 0;
}