COMPILER - Editorial

Author: Bruno Oliveira
Tester: Sergey Kulik
Editorialist: Lalit Kundu

EASY

PROBLEM:

Given a bracket sequence, print the length of largest prefix that is a regular bracket sequence.

EXPLANATION:

A regular bracket sequence is defined as follows:

1. S="" is regular.
2. S="<" + S1 + “>” is regular, if S1 is regular.
3. S=S1 concat S2 is regular, if S1 and S2 are regular.

If S is regular bracket sequence, for any i, number of closing brackets in S[0,i] should not exceed number of opening brackets. Also, if number of opening brackets is equal to number of closing brackets in S[0,i], S[0,i] is a regular bracket sequence.

``````def check(s):
t=0,ans=0
for i=0 to N-1:
if s*=='<': t++;
else:
t--;
//Now, if t=0, means, the string s[0,i] is valid.
if t==0: ans=max(ans,i+1)
else if t<0: break   //string s whole is invalid.
print ans
``````

Complexity: O(N).

AUTHOR’S AND TESTER’S SOLUTIONS:

9 Likes

Can anyone tell me why this code gives WA??

can i get sometest cases want to figure out where i am getting wrong, comipler of codeshef was flashing WRONG ANSWER

Can anyone tell me why my this Pascal code was giving wrong ans but same implementation in C++ passed ?

Can somebody please tell me the mistake that I did in this C code: http://ideone.com/HcmfNJ

Got a WA… :’(

Can anyone post a java implementation for this?

Incidentally a similar question was asked on forums a few month’s back.

1 Like

Can anyone tell which case i am leaving ??
Have tried many times but cant get it Accepted.

Please give a test case where my code fails for compilers and parsers
[http://www.codechef.com/viewsolution/3876116] the above link is the code where i had written in java

why im getting WA for this.
http://www.codechef.com/viewsolution/3900904

what do you mean by ad-hoc? ( one of prerequisites)

Can someone provide a solution for the same above problem if the word prefix was removed. I am getting considerable difficulties in doing this version. Eg. for <<<<<<<<> output:2 , for <><><<<<<<<<<> output:4.
My problem is that how do we track whether the new contiguous sub sequence extends the previous one or not. As in <<>><<>> output:8 but for <<>><<<>> output:4.

you can solve the problem using divide and conquer approach with a segment tree. At each node of the segment tree, store the following 5 variables:

l -> the maximum positive sum from right end

li -> the index at which the maximum sum is achieved

r -> the minimum negative sum from left end

ri -> the index at which this minimum sum is achieved

b -> longest perfect bracket sequence in the interval

So, for a node N, the best can be calculated as the maximum of b values of the children, or taking the joint of the r value of the right child and l value of the left child. if mod of l of left child is less than mod of r of right child, then just find the index in the right child where the prefix sum is equal to -l. similar procedure if mod of r is smaller than mod of l. this is work in NlogN.

http://www.codechef.com/viewsolution/4150026 .WA checked with all cases discused above .Thanks in advance

For which test case am i getting WA ?. Please someone answer . my subbmission http://www.codechef.com/viewsolution/4167497 Thanks in advance

What must be the output of ><<>> …Should it be 0 or 4 ??

1 Like

Why do we need to calculate “ans=max(ans,i+1)”? We can just write “ans=i+1” which will also work.

Terima kasih dan salam kenal.

https://www.codechef.com/viewsolution/15906400
i dn know why i am getting WA please anyone suggest some edge cases .

Can anyone tell me what’s wrong with my code ?

Thanks !