Given a string, find if there is any subsequence of length 2 or more that repeats itself such that the two subsequences doesn’t have the same character at the same position, i.e., any 0’th or 1st character in the two subsequences shouldn’t have the same index in the original string.
Input: ABCABD Output: Repeated Subsequence Exists (A B is repeated) Input: ABBB Output: Repeated Subsequence Exists (B B is repeated) Input: AAB Output: Repeated Subsequence Doesn't Exist (Note that A B cannot be considered as repeating because B is at same position in two subsequences). Input: AABBC Output: Repeated Subsequence Exists (A B is repeated) Input: ABCDACB Output: Repeated Subsequence Exists (A B is repeated) Input: ABCD Output: Repeated Subsequence Doesn't Exist
In this post, O(n) time and space approach is discussed.
The idea is to remove all the non-repeated characters from the string and check if the resultant string is palindrome or not. If the remaining string is palindrome then it is not repeated, else there is a repetition. One special case we need to handle for inputs like “AAA”, which are palindrome but their repeated subsequence exists. Repeated subsequence exists for a palindrome string if it is of odd length and its middle letter is same as left(or right) character.
Below is the implementation of above idea.
Repeated Subsequence Exists
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