P4513 小白逛公园 动态维护最大子段和

题目链接：https://www.luogu.org/problem/P4513

#include<iostream>
#include<cstdio>
#include<algorithm>
using namespace std;
#define maxn 500005
#define ll long long
#define ls l,mid,rt<<1
#define rs mid+1,r,rt<<1|1
struct node{
    ll sum,lsum,rsum,msum;
}tr[maxn<<];
ll a[maxn];
inline void pushup(int rt)
{
    tr[rt].sum=tr[rt<<].sum+tr[rt<<|].sum;
    tr[rt].lsum=max(tr[rt<<].lsum,tr[rt<<].sum+tr[rt<<|].lsum);
    tr[rt].rsum=max(tr[rt<<|].rsum,tr[rt<<|].sum+tr[rt<<].rsum);
    tr[rt].msum=max(max(tr[rt<<].msum,tr[rt<<|].msum),tr[rt<<|].lsum+tr[rt<<].rsum);
}
inline void build(int l,int r,int rt)
{
    if(l==r)
    {
        tr[rt].sum=tr[rt].msum=tr[rt].lsum=tr[rt].rsum=a[l];
        return ;
    }
    int mid=l+r>>;
    build(ls);build(rs);
    pushup(rt);
}
inline void update(int L,int c,int l,int r,int rt)
{
    if(l==r)
    {
        tr[rt].sum=tr[rt].msum=tr[rt].lsum=tr[rt].rsum=c;
        return ;
    }
    int mid=l+r>>;
    if(L<=mid)update(L,c,ls);
    else update(L,c,rs);
    pushup(rt);
}
inline node query(int L,int R,int l,int r,int rt)
{
    if(L<=l&&R>=r)return tr[rt];
    int mid=l+r>>;
    if(R<=mid)return query(L,R,ls);
    else if(L>mid)return query(L,R,rs);
    else
    {
        node wl=query(L,R,ls),wr=query(L,R,rs),ans;
        ans.lsum=max(wl.lsum,wl.sum+wr.lsum);
        ans.rsum=max(wr.rsum,wr.sum+wl.rsum);
        ans.msum=max(max(wl.msum,wr.msum),wl.rsum+wr.lsum);
        return ans;
    }
}
int main()
{
    int n,m;
    scanf("%d%d",&n,&m);
    for(int i=;i<=n;i++)
    scanf("%lld",&a[i]);
    build(,n,);
    int op,l,r,x;
    for(int i=;i<=m;i++)
    {
        scanf("%d",&op);
        if(op==)
        {
            scanf("%d%d",&l,&r);
            if(l>r)swap(l,r);
            printf("%lld\n",query(l,r,,n,).msum);
        }
        else
        {
            scanf("%d%d",&l,&x);
            update(l,x,,n,);
        }
    }
    return ;
}