POSTERS - Election Posters

A parliamentary election was being held in Byteland. Its enterprising and orderly citizens decided to limit the entire election campaign to a single dedicated wall, so as not to ruin the panorama with countless posters and billboards. Every politician was allowed to hang exactly one poster on the wall. All posters extend from top to bottom, but are hung at different points of the wall, and may be of different width. The wall is divided horizontally into sections, and a poster completely occupies two or more adjacent sections.

With time, some of the posters were covered (partially or completely) by those of other politicians. Knowing the location of all the posters and the order in which they were hung, determine how many posters have at least one visible section in the end.


The input begins with the integer t, the number of test cases. Then t test cases follow.

Each test case begins with a line containing integer n - the number of posters (1 <= n <= 40000). Then n lines follow, the i-th (1 <= i <= n) containing exactly two integers li ri, denoting the numbers of the leftmost and rightmost sections covered by the i-th poster (1 <= li < ri <= 107). The input order corresponds to the order of hanging posters.


For each test case output a line containing one integerĀ - the number of posters with visible sections.


Sample input:
1 4
2 6
8 10
3 4
7 10

Sample output:

An illustration of the sample input is given below.

The wall with posters

hide comments
kumarpritam863: 2020-09-07 11:07:45

co-ordinate compression is the easiest way to solve although can be solved using lazy propagation and dsu.

geek_goku6: 2020-07-26 22:10:16

Can someone tell me how to do this problem with lazy propagation. Email id:

Last edit: 2020-07-27 13:58:12
thanos_tapras: 2020-06-15 21:36:35

Nice problem!
Many people suggest lazy propagation. However, I find it much more hassle-free to solve it using coordinate-compression.

nafisa_naznin: 2020-06-11 14:25:10

@fahimcp495. You said,
it seems to me that the judge solution is not right according to the problem statement.
i.e. for
20 30
10 20
30 40

output should be 3 but AC code gives 2

I think the judge solution is wrong. It will give 2 for compression. Because then 10 20 will become 2 - 3. But this 2 will be covered by 1 - 10 (1-2 after compressing) and 3 will be covered by 20-30 (3 -4 after compressing). That's because 10 and 20 has a 10 gap in it. But after compression 2 and 3 are adjacent. And nothing is in between them. For this problem, I think we should take compress array with some gaps before and after it. As if the array is [1 10 20 30] then we should consider it as [0 1 2 9 10 11 19 20 21 29 30 31]. Then we can cut off the duplicate or invalid ones.

fahimcp495: 2020-06-03 02:33:17

it seems to me that the judge solution is not right according to the problem statement.
i.e. for
20 30
10 20
30 40

output should be 3 but AC code gives 2

masterchef2209: 2020-04-07 09:38:35

nice lazy propogation,segtree+coordinate compression

aayush_b1999: 2020-03-28 12:52:13

can be solved using coordinate compression and sqrt decomposition

chinhchamhoc: 2019-11-29 04:22:32

could someone tells me how to solve using DSU pls

hduoc2003: 2019-09-28 18:08:29

Weak test cases.
segtree + lazy = 0.02s
dsu = 0.01s
But i used co-ordinate compression for both :D.
Can anyone tell me how to do lazy without compressing pls :))

Last edit: 2019-09-28 18:09:26
prmondal: 2019-07-01 17:39:21

Last edit: 2019-07-05 08:55:25

Added by:adrian
Time limit:7s
Source limit:50000B
Memory limit:1536MB
Cluster: Cube (Intel G860)
Resource:VI Polish Collegiate Team Programming Contest (AMPPZ), 2001