DMOPC '21 Contest 8 P6 - Castle Building
View as PDFBilly is playing with his newly bought Lego set containing  blocks with distinct heights from 
 to 
. He would like to build a castle with these blocks, which involves arranging the blocks in a row where the heights from left to right are 
 such that there is some index 
 
 where the heights are strictly increasing from 
 to 
 and strictly decreasing from 
 to 
. Formally, 
.
To help guide him, Billy found an old, worn out instruction manual from the bottom of his bed. The instructions give the required heights of each building block  from left to right. However, some of these numbers are so smudged that he cannot determine what they are! These are denoted with 
. Furthermore, Billy revises his interpretation of the instructions 
 times, where he assumes that the number at index 
 is actually 
.
Perplexed, Billy runs to you for help. Initially and after each of the  revisions, tell Billy if there exists a castle consistent with the instructions.
Constraints
Initially and after each of the  revisions, the instructions contain no duplicates of positive values.
Subtask 1 [30%]
Subtask 2 [40%]
Subtask 3 [30%]
No additional constraints.
Input Specification
The first line contains  integers 
 and 
.
The next line contains  integers, the initial instructions 
.
Each of the next  lines contains the description of a revision, 
 integers 
 and 
.
Output Specification
Initially and after each of the  revisions, output 
YES if there exists a castle consistent with the instructions and NO otherwise.
Sample Input
9 3
1 0 0 5 0 0 8 0 2
5 9
7 0
5 0
Sample Output
YES
NO
YES
YES
Explanation
Initially, 1 3 4 5 7 9 8 6 2 is a possible castle.
After the first revision, the instructions are 1 0 0 5 9 0 8 0 2, which has no consistent castle.
After the second revision, the instructions are 1 0 0 5 9 0 0 0 2, and 1 3 4 5 9 8 7 6 2 is a possible castle.
After the third revision, the instructions are 1 0 0 5 0 0 0 0 2, which has a few possible castles.
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