Palindrome Pairs

Question

Given a list of unique words, find all pairs of distinct indices (i, j) in the given list, so that the concatenation of the two words, i.e. words[i] + words[j] is a palindrome.

Example 1:
Given words = ["bat", "tab", "cat"]
Return [[0, 1], [1, 0]]
The palindromes are ["battab", "tabbat"]


How can I solve this using Trie data structure?

Why do you want to use Trie specifically? Sort the input word and store it a hashmap and the next time you encounter a sorted word you know that it is a palindrome pair. — thebenman, May 09 '18 at 03:22

score 1 · Answer 1 · answered May 09 '18 at 09:13

With Brute Force you can easily find all palindromes in O(N²K) where N is the number of the words in the list and K is the maximum length that is checked for a palindrome.

Using Trie You you can achieve your results in O(NK²).

Pseduocode

1) Create an empty Trie.
2) Do following for every word:-
    a) Insert reverse of current word.
    b) Also store up to which index it is 
       a palindrome.
3) Traverse list of words again and do following 
   for every word.
    a) If it is available in Trie then return true
    b) If it is partially available
         Check the remaining word is palindrome or not 
         If yes then return true that means a pair
         forms a palindrome.
         Note: Position upto which the word is palindrome
               is stored because of these type of cases.

Reference: Palindrome Pairs

Palindrome Pairs

1 Answers1