ORACLCS - Editorial

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Contest
Practice

Author: Praveen Dhinwa
Tester: Sergey Kulik
Editorialist: Kevin Atienza

PREREQUISITES:

longest common subsequence

PROBLEM:

You are given n strings, each of length m and consisting only of the characters a or b. Find another such string such that the longest common subsequence (LCS) of all n+1 strings is minimized. Output only this smallest LCS.

QUICK EXPLANATION:

The smallest LCS is always a constant string, i.e. either aaa...a or bbb...b. Thus the answer is the minimum number of as or bs in any string.

EXPLANATION:

First, we introduce a little bit of notation. Let s be a string of length, say, m. We write this as |s| = m. For 0 \le i \le m, we denote by s* the $i$th character of s, and s_i the prefix of s consisting of the first i characters. Thus, s_m = s, and s_0 = \varepsilon, the empty string. As an example, suppose s is codechef. Then s_4 is code and s[4] is c.

Longest common subsequence

The problem of finding the longest common subsequence (LCS) is quite standard, and there is a well known (dynamic programming) algorithm for it: Suppose we want to find the length of the LCS of two strings of length m, say s and t. Let f(a,b) be the LCS of s_a and t_b. Then (the length of) the LCS of s and t themselves is f(m,m). The following gives one way to compute f recursively:

  • As base cases, we have f(a,b) = 0 if a = 0 or b = 0, because in these cases, one of the strings is empty.
  • If a > 0, b > 0 and s[a] = t**, then f(a,b) = f(a-1,b-1) + 1.
  • If a > 0, b > 0 and s[a] ot= t**, then f(a,b) = \max(f(a-1,b), f(a,b-1)).

By tabulating all (m+1)^2 possible sets of arguments to f and filling the table in the right order, one can achieve an O((m+1)^2), or O(m^2), time algorithm to compute (the length of) the LCS of two strings.

This algorithm can be extended to more strings. However, the complexity becomes greater and greater the more strings you add. For example, for three strings, you would need a three-dimensional array for f, so the running time is O(m^3). In general, to compute the LCS of n strings this way would require O(nm^n) time, which is not polynomial-time. In fact, the problem of computing the LCS of a set of strings is NP-complete!

It’s not even clear how to brute-force the problem to pass the first subtask. One idea might be to try all binary strings of length m, and for each string, compute its LCS together with the other n strings. To compute the LCS, one can use the algorithm above. But there are n+1 strings, so the complexity is O(nm^{n+1}), which is too slow for the worst case m = n = 14.

A faster way would be to simply try all 2^{m+1} - 1 possible LCSes, i.e. for each string of length at most m, check if it is a subsequence of all strings, and get the longest. Checking subsequences can be done in linear time, so this check runs in O(mn), which is much more acceptable.

However, since there are 2^{m+1}-1 strings to try, this procedure will be called 2^{m+1}-1 times and so the overall complexity is O(mn 2^m), which is too large even for the smallest subtask!

n = 1

Let’s start small. Suppose n = 1, that is, suppose there is only one string of length m. Our goal is to find another such string such that the length of the LCS is minimized. Clearly this is a special case of the current problem and so we must be able to solve this case.

As an example, suppose the string we have is bbbbaaaaaa. Let’s try to concoct a string that minimizes the LCS.

Let’s try a simple candidate: aaaaaaaaaa. What is the LCS? Obviously it’s aaaaaa, and the length is 6. So we now have an upper bound for the answer: 6. But can we do better?

To do better, the first thought is that you can’t use too many as, because otherwise you might have aaaaaa as a common subsequence again, so we’d have failed to improve on our bound. In fact, why don’t we try avoiding any as at all? Let’s try the string bbbbbbbbbb. This way, the LCS is bbbb, which only has length 4. We now have a better upper bound! But again, can we do better?

In order to do better, similar to the a case, we can’t use more than three bs, otherwise bbbb will be a common subsequence, and we wouldn’t have improved on our best solution. This means that we can use at most 3 bs. But if there are only up to 3 bs, then there are at least 7 as, which means aaaaaa will be a common subsequence. And this is longer than bbbb! Thus, we have just shown that it’s impossible to improve on the smallest LCS of 4, and in fact the smallest LCS for this case is 4.

The nice thing about this argument is that it easily works for any other string! More specifically:

  • Suppose our string has a as and b bs. (Thus a + b = m.)
  • We can force the LCS to be \min(a,b) in length, by using the string aaa...a or bbb...b, whichever of a or b is smaller respectively.
  • But we can show that we can’t do any better than \min(a,b). Because suppose we found another string of length m with LCS less than \min(a,b). This string cannot have \min(a,b) as, or \min(a,b) bs, otherwise the LCS would have been \min(a,b) (or longer). Thus, there are less than \min(a,b) as and bs in this string. But this means that the total length of the string is less than \min(a,b) + \min(a,b) \le a + b = m, a contradiction.
  • Thus, the answer is \min(a,b).

Thus, we have just shown that answer for a single string is \min(a(s),b(s)), where a(s) and b(s) are the number of as and bs in s, respectively.

n > 1

Now, what if n > 1? Similarly to the above, it’s immediately clear that we have the following upper bound:

\min_{ ext{string $s$}} \min(a(s),b(s))

Where s runs across all n strings in the input. This LCS is achieved by trying the strings aaa...a and bbb...b. Now, this time can we do better?

In fact, we can easily adjust the argument above to show that this is the smallest LCS we can get!

  • We can force the LCS to be C = \min_s \min(a(s),b(s)) in length, by using the string aaa...a or bbb...b, whichever yields the smaller LCS. Note that a(s) + b(s) = m for any string s.
  • But we can show that we can’t do any better than C. Because suppose we found another string of length m with LCS less than C. This string cannot have C as, or C bs, otherwise the LCS would have been C (or longer). Thus, there are less than C as and bs in this string. But this means that the total length of the string is less than C + C \le a(s) + b(s) = m (for any string s), a contradiction.
  • Thus, the answer is C = \min_s \min(a(s),b(s)).

The value \min_s \min(a(s),b(s)), can be computed by simply counting the as and bs in each string! This runs in linear time in the input.

Here’s an implementation in Python:

for cas in xrange(input()):
    A = B = 10**9
    for i in xrange(input()):
        s = raw_input()
        A = min(A, sum(c == 'a' for c in s))
        B = min(B, sum(c == 'b' for c in s))
    print min(A, B)

Here’s a shorter (and codegolfier) implementation:

from collections import Counter
for cas in xrange(input()):
    print min(min(Counter(raw_input()+'ab').values()) for i in xrange(input()))-1

Bonus question: Does the above algorithm still work if the alphabet were larger than \{a,b\}?

Time Complexity:

O(nm)

AUTHOR’S AND TESTER’S SOLUTIONS:

Setter
Tester
Editorialist

1 Like

What should be the output for this ?

1
2
aabb
aabbb

Just 10 lines of coding,really enjoyed the problem. Here’s my code.

Why is the complexity for brute-force approach O(mn4^m)? 2^m different strings has already been accounted for.

So, for one test case the overall complexity seems to be O(2^mmn), because everytime we try matching all {a,b}^m strings to n strings of length m.

1 Like

great editorial, mate :slight_smile:

2 Likes

Mayank

hi… i visited first time here nd find a very good articles related to truth quotes

truth quotes

2 @debverine

@arpn shouldnt it be 4 ?? (as for “aabb”)

yea,but hidden string could be aaaa or bbbb,in that case answer would be 2. Because LCS(aabb,aaaa)=2.

@debverine

Test case is wrong here. Length of each string in test case should be same.

You are correct. Thanks for pointing it out. I have updated the editorial.