Given two strings A and B. The task is to count the number of ways to insert a character in string A to increase the length of the Longest Common Subsequence between string A and string B by 1.
Input : A = “aa”, B = “baaa”
Output : 4
The longest common subsequence shared by string A and string B is “aa”, which has a length of 2.
There are two ways that the length of the longest common subsequence can be increased to 3 by adding a single character to string A:
- There are 3 different positions in string A where we could insert an additional ‘a’ to create longest common subsequence “aaa” (i.e at the beginning, middle, and end of the string).
- We can insert a ‘b’ at the beginning of the string for a new longest common subsequence of “baaa”. So, we have 3 + 1 = 4 ways to insert an alphanumeric character into string A and increase the length of the longest common subsequence by one.
Let say for a given string A and string B, the length of their LCS is k. Let’s insert a single character ‘c’ after the ith character in string A and denote the string formed after the insertion as string Anew, which looks like:
Anew = A1, i . c . Ai + 1, n
where Ai, j denotes a substring of string A from the ith to the jth characters and ‘.’ denotes a concatenation of two strings.
Let’s define knew to be the length of the LCS of Anew and B. Now we want to know if knew = k + 1.
The crucial observation is that the newly inserted character ‘c’ must be a part of any common subsequence of Anew and B having length > k. We know this because if there is any common subsequence of Anew and B, this is a contradiction because it would mean the length of the LCS of A and B is > k.
Using the above observation, we can try the following approach. For each possible character ‘c'(there are 52 upper and lower case English letters and 10 Arabic digits, so there are 62 possible characters to insert) and for every possible insertion i in String A (there are |a| + 1 insertion positions), let’s try to insert ‘c’ after the ith character in string A and match it with every occurrence of ‘c’ in string B, we can try to match these ‘c’ characters such that:
A1, i . c . Ai+1, n
B1, j-1 . c . Bj+1, m
Now, in order to check if such an insertion produces an LCS of length k + 1, it’s sufficienet to check if the length of the LCS of A1, i and B1, j-1 plus the length of the LCS Ai+1, n and Bj+1, m is equal to k. In this case, the lCS of Anew and B is k + 1 because there is both a match between the fixed occurances of character ‘c’ and there is no longer common subsequence between them.
If we can quickly get the length of the LCS between every two prefixes of A and B as well as between every two of their suffixes, we can compute the result. The length of the LCS between their prefixes can be read from a Dynamic Programming table used in computing the LCS of string A and string B. In this method, dp[i][j] stores the length of longest common subsequence of A, i and Bi, j. Similarly, the length of the LCS between their suffixes can be read from an analogous dp table which can be computed during computation of the LCS of Areversed and Breversed where Sreversed denotes the reversed string S.
Time Complexity: O(N x M)
- Count ways to increase LCS length of two strings by one
- Insert a Character in a Rotated String
- Ways to split string such that each partition starts with distinct character
- Number of ways to represent a number as sum of k fibonacci numbers
- Number of ways to get a given sum with n number of m-faced dices
- Count number of ways to get Odd Sum
- Number of ways to pair people
- Number of ways to swap two bit of s1 so that bitwise OR of s1 and s2 changes
- Number of ways to reach the end of matrix with non-zero AND value
- Count the number of ways to traverse a Matrix
- Count number of ways to partition a set into k subsets
- Count number of ways to cover a distance
- Bell Numbers (Number of ways to Partition a Set)
- Number of ways to arrange N items under given constraints
- Count number of ways to jump to reach end
If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to firstname.lastname@example.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please Improve this article if you find anything incorrect by clicking on the "Improve Article" button below.