CHANOQ - Editorial

chanoq
editorial
feb18
medium-hard
persistence
segment-tree
sqrt-decomp

#1

PROBLEM LINK:

Practice
Contest

Author: Mark Kornejchik
Tester: Hanlin Ren
Editorialist: Hanlin Ren

DIFFICULTY:

MEDIUM-HARD

PREREQUISITES:

persistent segment tree, sqrt decomposition

(Note: I can’t find wikipedia articles for persistent segment tree, but this seems like a good tutorial to read. You can also google “persistent segment tree” for this topic. The editorial assumes familiarity with persistent segment tree.)

PROBLEM:

There are n intervals [l_i,r_i] where 1\le l_i\le r_i\le n. You have Q queries, where each query is given CNT distinct points x_1,x_2,\dots,x_{CNT}(1\le x_i\le n), and you need to output the number of good intervals. An interval is good if it crosses an odd number of query points. An interval [l,r] cross a point x if l\le x\le r.

QUICK EXPLANATION:

For a query of CNT points:

  • if CNT\ge \sqrt{\frac{n}{\log n}}, then we use an O(n)-time brute force: we can count, for each interval [l_i,r_i], the number of points that it crosses, in a total time of O(n). Then we simply report the answer.
  • if CNT<\sqrt{\frac{n}{\log n}}, then we use the following O(CNT^2\log n) algorithm: first we sort the points by ascending order, say they are x_1,x_2,\dots,x_{CNT}. We then enumerate the first and the last point x_i,x_j that’s crossed by the interval, ensuring that j-i is even(so there are odd points crossed); and look up how many intervals satisfy this condition. For an interval [l,r], this condition is equivalent to x_{i-1}+1\le l\le x_i and x_j\le r\le x_{j+1}-1, which can be maintained in a data structure to support O(\log n)-time queries.

EXPLANATION:

subtask 1

This subtask can be solved by straightforward brute force in O(N\cdot sum(CNT))=O(N^2) time.

subtask 2

a special case: CNT=1

In this special case, each query is a point x\in[1,N], and we’d like to find the number of intervals which crosses x. However, we’ll consider the following form of query, which will be useful later: given L_1,R_1,L_2,R_2, how many intervals [l_i,r_i] are there such that L_1\le l_i\le R_1 and L_2\le r_i\le R_2?

To answer such query, let’s build a persistent segment tree. The x-th root is a segment tree, and for a node which represents [L,R] in this tree, it stores the number of intervals with l_i\le x and r_i\in[L,R]. It can be constructed by the following pseudocode:

sort the intervals such that l_1<=l_2<=...<=l_n
j = 1
for i = 1 to n
	(root of i-th tree) = (root of (i-1)-th tree)
	while (l_j <= i)
		insert r_j into (root of i-th tree)
		j = j + 1

Let f(a,L_2,R_2) be the number of intervals with l_i\le a and L_2\le r_i\le R_2. We can query f(a,L_2,R_2) in O(\log n) time: we simply query the sum of [L_2,R_2] in the a-th tree. Then we can answer the above query (L_1,R_1,L_2,R_2): the answer is simply f(R_1,L_2,R_2)-f(L_1-1,L_2,R_2).

the solution

We set a parameter S=O(\sqrt{\frac{n}{\log n}}).

If CNT\le S, then we sort the points as x_1 < x_2 < \dots < x_{CNT}, and enumerate L,R such that 1\le L\le R\le CNT and R-L is even. We can find the number of intervals such that [l_i,r_i]\cap \{x_1,\dots,x_{CNT}\}=\{x_j:j\in [L,R]\}. This is simply done by a query (x_{L-1}+1,x_L,x_R,x_{R+1}-1). This takes O(CNT^2\log n)=O(CNT\sqrt{n\log n}) time.

If CNT>S, the query can be answered by brute force: we let s* equal to the number of x_j's such that x_j\le i, and the number of points [l_i,r_i] crosses is just s[r_i]-s[l_i-1]. This takes O(n)=O(CNT\sqrt{n\log n}) time.

The total time complexity is O(n\sqrt{n\log n}).

ALTERNATIVE SOLUTION:

Please feel free to share your approaches :slight_smile:

AUTHOR’S AND TESTER’S SOLUTIONS:

Author’s solution can be found here.
Tester’s solution can be found here.


#2

I solved it using bitsets.
For each N, calculate the intervals it belongs to and make a bitset corresponding to the index of the interval.

For the given example.

  • interval 0 = (4 5)
  • interval 1 = (3 5)
  • interval 2 = (2 4)
  • interval 3 = (1 3)
  • interval 4 = (5 5)

The bitsets for each N would be

  1. 00010
  2. 00110
  3. 01110
  4. 11100
  5. 11001

Now for each element in the query cumulatively xor all the bitsets. And at the last count all the bits that are set. This would give the number of intervals that contain odd number of integers.

I read somewhere bitset uses some hashing mechanism to compute the operations. Thus this passes. If testcases could be made so that hashing collides most of the times, this won’t work, as it would take O(n) to only count the number of bits set.

View Solution


#3

For those who are having difficuly in understanding the persistent seg tree solution, see my easier approach. I have used MO’s algo for solving it. Do have a look https://discuss.codechef.com/questions/122723/chanoq-unofficial-editorial-chef-and-odd-queries-feb-long


#4

Alternately, Only Segment Trees can be used to solve the problem by offline processing the queries in case of small M and binary search when M is large.


#5

I didn’t used exact brute force, but used frequency array where freq* stores the number of numbers less than i.
But got TLE in 4 tests.


[1]
<br>Any optimization possible for this approach @admin, @r_64 @vijju123. Or Was Segment tree/Sqrt root decomposition only required approach for solving this problem?

  [1]: https://www.codechef.com/viewsolution/17280411

#6

I understood lovemaydie’s solution better than any other editorialist’s explanation… very good @lovemaydie … nice skill of explaining… official editorial and method are damn hard to understand…


#7

@admin, Author’s solution is showing Access Denied. Please fix it.


#8

I wrote the same thing as author but I kept getting tle. I think it’s because I implemented persistent segment tree first time. Can anyone help me with optimization.
Link https://www.codechef.com/viewsolution/17421721


#9

@vivek_shah r_64 will not reply … bare log hai bhaiya … and he also doesn’t get any benifit from replying to u …>> ask others


#10

Persistent Segment Trees @ https://blog.anudeep2011.com/persistent-segment-trees-explained-with-spoj-problems/


#11

I understood the editorial but how did arrive at the parameter \sqrt{(n/logn)} what is derivation?


#12

I solved this problem using bitset in contest time.
https://paste.ubuntu.com/p/sZSB6rDsyQ/


#13

can someone plz show me a solution based on mergesort tree plz


#14

The author didn’t write his solution. >_>


#15

Yup, the author did not write his solution. He thought that somebody will get full 100 points- he will write his solution based on that approach. Thanks @r_64 for confirmation :slight_smile: :3


#16

If author didn’t write any solution then how did he generate the testcases ? :stuck_out_tongue:


#17

I think merge sort tree would probably give TLE because for each query it will take (logN*logN).


#18

He could have requested the tester to solve it and send his model solution for him to “check/validate” :3 :stuck_out_tongue: @abx_2109


#19

Oh…so only tester wrote a solution and that solution was used to generate the output files and the same solution was used to submit the problem? So technically no one tested the problem. Smart author I must say xD


#20

Can u reply to above @r_64, @vijju123. I am trying to upsolve this question…