Finding Maximal 2-Dimensional Palindromes

This paper extends the problem of palindrome searching into a higher dimension, addressing two definitions of 2D palindromes. The first definition implies a square, while the second definition (also known as a centrosymmetric factor), can be any rectangular shape. We describe two algorithms for searching a 2D text for maximal palindromes, one for each type of 2D palindrome. The first algorithm is optimal; it runs in linear time, on par with Manacher’s linear time 1D palindrome algorithm. The second algorithm searches a text of size n1 × n2 (n1 ≥ n2) in O(n2) time for each of its n1 × n2 positions. Since each position may have up to O(n2) maximal palindromes centered at that location, the second result is also optimal in terms of the worst-case output size. 1998 ACM Subject Classification F.2.2 Nonnumerical Algorithms and Problems