线段树最大子段

• https://codeforces.com/contest/1359/problem/D

线段树最大子段模板

struct node{
	int l,r;
	int sum,ms;//maxsum
    int ml,mr;//maxl,maxr
}tree[N*4];


void PushUp(int i)
{
	tree[i].sum=tree[i<<1].sum+tree[i<<1|1].sum;
	tree[i].ml=max(tree[i<<1].sum+tree[i<<1|1].ml,tree[i<<1].ml);
	tree[i].mr=max(tree[i<<1|1].sum+tree[i<<1].mr,tree[i<<1|1].mr);
	tree[i].ms=max(max(tree[i<<1].ms,tree[i<<1|1].ms),tree[i<<1].mr+tree[i<<1|1].ml);
}
void build(int i,int l,int r)
{
	tree[i].l=l,tree[i].r=r;
	if(l==r)
	{
		tree[i].sum=tree[i].ml=tree[i].mr=tree[i].ms=a[l];
		return ;
	}
	int mid=(l+r)>>1;
	build(i<<1,l,mid);
	build(i<<1|1,mid+1,r);
	PushUp(i);
}
void update(int i,int pos,int val)
{
	if(tree[i].l==tree[i].r)
	{
		tree[i].ms=tree[i].ml=tree[i].mr=tree[i].sum=val;
		return ;
	}
	int mid=(tree[i].l+tree[i].r)>>1;
	if(pos<=mid)
		update(i<<1,pos,val);
	else update(i<<1|1,pos,val);
	PushUp(i);
}
node query(int i,int l,int r)
{
	if(l<=tree[i].l&&tree[i].r<=r)
		return tree[i];
	int mid=(tree[i].l+tree[i].r)>>1;
	if(r<=mid) return query(i<<1,l,r);
	else if(l>mid) return query(i<<1|1,l,r);
	else
	{
		node x=query(i<<1,l,r),y=query(i<<1|1,l,r),res;
		//合并答案 
		res.sum=x.sum+y.sum;
		res.ml=max(x.sum+y.ml,x.ml);
		res.mr=max(y.sum+x.mr,y.mr);
		res.ms=max(max(x.ms,y.ms),x.mr+y.ml);
		return res;
	}
}

参考