TACHEMIS - Editorial

Problem Link:

Practice

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Difficulty:

Medium

Pre-requisites:

Manacher’s Algorithm

Problem:

Given a string in “compressed notation”(see problem), find the number of substrings that are palindrome.

Explanation:

The problem was a straightforward application of Manacher’s Algorithm. This Algo was originally proposed to find the longest palindrome in a string. However, for each “center” c, it finds the length of longest palindrome “centered” at c. Therefore, we can use it to find the number of palindromes “centered” at c as well. Handling the fact that strings are given in compressed notation is relatively straightforward.

Many elegant descriptions of Manacher’s Algorithm can be found on internet.

In very short, Manacher’s Algorithm can be written as follows.


1 | int p[N+1], mx = 0, id = 0;
2 | // length of longest palindrome centred at i is 2 * p[i]-1.
3 | for (i = 1; i <= N; i++) {
4 |     p[i] = mx > i ? min(p[2 * id-i], mx-i) : 1;
5 |     while (s[i + p[i]] == s[i - p[i]]) p[i]++;
6 |     if (i + p[i] > mx) {
7 |         mx = i + p[i];
8 |         id = i;
9 |     }
10| }

Let the compressed string be (c1, k1), (c2, k2), … (cN, kN).

Assuming reader understands Manacher’s Algorithm, here is how to modify it for this problem:

  • Palindromes that contain only a single character repeated several times can be counted as:

Σ ki * (ki +1) / 2

  • Palindromes that span over more than 1 contiguous segment of compressed string can be computed by also maintaining an array q, which stores the length of the longest “decompressed” palindrome centred at ith segment.

q[i] = ki-p[i]+1 + ki-p[i]+2 … + ki + … ki+p[i]-2 + ki+p[i]-1

There will also be minor changes like:

  1. We need not put interleaving '#'es because center of every palindromic substring is the center of some “compressed” segment(unless it fully lies inside a segment).
  2. In line no 5, compare the characters as well as the lengths of the segments.
  3. If the segments i-p[i] and i+p[i] do not have same lengths, but have the same character, then q[i] would need to adjusted by adding 2*min(ki-p[i], ki+p[i]).

The final Answer is

Σ ki * (ki +1) / 2 + ⌈q[i]/2⌉ - ⌈ki/2⌉

Setter’s Solution:

Can be found here

Tester’s Solution:

Can be found here

8 Likes

Can Anybody Tell Why My Answer didn’t got Accepted?
My Solution id is :2397163
I used the Same Algorithm

I cannot see that straightforward application. Do you “decompress” the string in memory? I think no, it’s too big. So what about compressed string

A3
B3
C2
B2
A3

In compressed string the substring ABCBA is palindrome, while it’s not valid palindrome in decompressed string…

I used hashing + binary search in order to find the longest palindrome centered at each of the K groups of identical characters. This gave me an O(K*log(K)) time complexity instead of O(K) (as is the case with Manacher’s algorithm), but I personally prefer to use hashing whenever possible, because it’s a much more general technique (applicable to a wider range of problems) and it’s very easy to implement.

10 Likes

Two ‘characters’ of compressed string are equal if the corresponding (char, int) pairs are equal. No need to explicitly decompress the string, just do calculations smartly.

1 Like

can anybody please identify why my code got SIGABRT error
http://www.codechef.com/viewsolution/2396428

I tried the same, but got WA for unknown reason. Going through your submission. :slight_smile:

In your solution, you calculate powers of 1e9+7 upto 200000 in an unsigned long long and also do the hash calculations in ULL. How is it that the ‘overflowed’ values don’t give an error and the algorithm still works ?

I wrote a very naive solution, which as I now see should most probably get TLE. But for some reason it gets WA. It works fine on any tests I can think of. Can anybody help find the reason for this WA? http://www.codechef.com/viewsolution/2397315

@sanchit_h because unsigned long long values automatically wraps around 2^64 on overflow.

1 Like

Oh. Thanks! Also, do you know why we need to a multiply the RHS value by a certain power(specifically 4*mid-2) while calculating the difference in the forward and reverse hash ?

Got my error. my hash function wasn’t good enough. using @mugurelionut’s hash func, got AC :confused:
@sanchit_h Haven’t seen that part of his code thoroughly. But I can guess that it is due to his nature of hash function. See my code if it makes any better for you.

@sanchit_h: My hash function considers that we have a sequence of 2N values: character1, count1, character2, count2, …, characterN, countN. That’s why I needed powers of P up to 2N basically (and not only up to N). Then, when computing the direct hash value for a substring (in my solution from i-mid+1 to i+mid-1) we need the “prefix” hash up to i+mid-1 from which we need to subtract the contribution of the “prefix” hash up to i-mid. But this contribution appears multiplied by P^(2 * length), where length=2*mid-1 (thus, 2 * length = 4 * mid - 2). The hash for the reverse string is similar.

2 Likes

My O(Klog(K)) solution got tle. Then, i solved it with manacher. May be because of using mods :frowning:

Change this :"(n*(n+1))/2" to “((long long) n*(n+1))/2” and you will get TLE.

1 Like

Your code would require O(10^9) memory, because it is trying to “expand” the compressed string.

1 Like

One reason is that arr is not valid after putting in all those '#'es. You should have updated arr as well.

http://www.codechef.com/viewsolution/2396623
plz have a look…
i know its not good enough but why wrong answer?

We cannot help you to debug your code, please only ask something related to the solution in the editorial!

@tuananh93 don’t be rude, of course he can ask for help

First thing is I do not understand how its possible that your code works when you read c[i] twice:

for(i=0;i<k;i++) {
	scanf("%c",&c[i]);
	scanf("%c%d",&c[i],&n[i]);
	res=res+(n[i]*(n[i]+1)/2);
}

but the problem is in n[i]*(n[i]+1)/2, try this test case:

1
1
A 1000000
2 Likes
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