O^O刚学线段树合并,有个小小的细节问题

int merge(int x,int y,int l,int r)
{
	if( x==0||y==0 )	return x+y;
	if( l==r ){ sum[x]+=sum[y]; return x; }
	ls[x] = merge(ls[x],ls[y],l,mid);
	rs[x] = merge(rs[x],rs[y],mid+1,r);
	pushup(x,l,r);
	return x;
}