[线段树] 基于最大子段和

https://www.luogu.com.cn/problem/SP1043

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cctype>
#define re register
#define ll long long
#define Seg SegmentTree
#define lson p<<1,l,mid
#define rson p<<1|1,mid+1,r
#define readd read<int>()
using namespace std;
const int maxn=5001000;
template<typename T> inline T& read(){
    T x=0;
    bool flag=false;
    register char ch=getchar();
    while(!isdigit(ch)&&ch!='-')
        ch=getchar();
    if(ch=='-')
        flag=true,ch=getchar();
    while(isdigit(ch))
        x=(x<<3)+(x<<1)+(ch^48),ch=getchar();
    if(flag)
        return x=(~x+1);
    else
    return x;
}
struct SegmentTree{
	int sum,qian,hou,dat;
}t[maxn<<2];
int n,m;
inline void update(int p){
	t[p].sum=t[p<<1].sum+t[p<<1|1].sum;
	t[p].qian=max(t[p<<1].qian,t[p<<1].sum+t[p<<1|1].qian);
	t[p].hou=max(t[p<<1|1].hou,t[p<<1|1].sum+t[p<<1].hou);
	t[p].dat=max(max(t[p<<1].dat,t[p<<1|1].dat),t[p<<1].hou+t[p<<1|1].qian);
}
inline void build(int p,int l,int r){
	if(l==r){
		t[p].dat=t[p].sum=t[p].qian=t[p].hou=readd;//到了叶子，一边读一边建树 
		return;
	}
	int mid=(l+r)>>1;
	build(lson);build(rson);
	update(p);//还是回溯时更新 
}
//主函数来了 
inline Seg query(int p,int l,int r,int x,int y){//查找赋值 
	if(x<=l&&y>=r)return t[p];
	int mid=(l+r)>>1;
	if(x>mid)return query(rson,x,y);
	if(y<=mid)return query(lson,x,y);//注意是<=
	else{
		Seg ans,a,b;
		a=query(lson,x,y);b=query(rson,x,y);
		ans.sum=a.sum+b.sum;
		ans.dat=max(a.dat,a.hou+b.qian),ans.dat=max(ans.dat,b.dat);
		ans.qian=max(a.qian,a.sum+b.qian);
		ans.hou=max(b.hou,b.sum+a.hou);
		return ans;
	}
}
int main(){
	n=readd;
	build(1,1,n);
	m=readd;
	for(re int i=1;i<=m;i++){
		int x=readd,y=readd;
		cout<<query(1,1,n,x,y).dat<<endl;
	}
	return 0;
}