Sometimes I encounter a type of range queries that I don't know how to do using segment tree, so I use a merge sort tree instead. It can answer queries in $$$O(log^2 n ) $$$. I decided to share this because it can be useful to many in contests. First, we know that a node in a segment tree stores the answer for a range of length $$$2^x$$$ for some $$$x$$$, but we can also store in it all the elements in the range sorted in a vector (using merge sort algo) or set or whatever. Here I am going to give a few examples how we can use this to our advantage.
Problem 1: Static Count of Lesser Elements range queries.
We are given an array $$$a_1, a_2, a_3... a_n$$$ and an integer $$$q$$$. The $$$i$$$_th of the next $$$q$$$ lines has three integers $$$l_i , r_i , x_i$$$. The answer for this query is how many $$$a_j$$$ such that $$$l_i \le j \le r_i$$$ and $$$a_j < x$$$.
Problem 2: Dynamic Lower Bound problem.
We are given an array $$$a_1, a_2, a_3... a_n$$$ and an integer $$$q$$$. The $$$i$$$_th of the next $$$q$$$ lines has an integer $$$t_i$$$, describing the type of query.
- If $$$t_i = 0$$$ then you are given two other integers, $$$idx_i$$$ and $$$x_i$$$, meaning that we should make $$$a_{idx} = x_i$$$.
- If $$$t_i = 1$$$ then you are given three integers $$$l_i, r_i, x_i$$$. You should answer with the smallest value $$$v$$$ in the range $$$[l_i,r_i]$$$ such that $$$x \le v$$$. If there is none, print -1.
Final notes
There are many other things that you can do using technique. You can even solve the dynamic version of the first problem using indexed_set. However you should use this only when you're desperate because it's really slow. Yes it's $$$O(nlog^2n)$$$ but the constant factor is very high because it's segment tree + sets. If you have intereseting problems to add to the blog, or you notice some mistake please say in the comments. Good luck everyone!
Thanks for your hard work on this blog..I really appreciate it. You deserve an upvote.
KQUERY
this problem uses above aproach but greater than x instead of less than x
For the first problem, we can use pbds (indexed multiset or similar) as a segtree node to update the values as well!
Yes, but it's static so vectors are faster
Very interesting! thx for sharing :D
You can actually improve the first sample problem complexity to $$$O$$$($$$logn$$$) per query with fractional cascading.
Could you provide a good blog to read about it?
The one in cp-algorithms.