Leetcode Palindrome Pairs

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"]

Example 2:
Given words = ["abcd", "dcba", "lls", "s", "sssll"]
Return [[0, 1], [1, 0], [3, 2], [2, 4]]
The palindromes are ["dcbaabcd", "abcddcba", "slls", "llssssll"]

Difficulty: Hard

Solution: Use Trie.

public class Solution {    Node root = new Node();    public void insert(String s, int index){        Node curr = root;        for(int i = s.length() - 1; i >= 0; i--){            if(curr.next[s.charAt(i)- 'a'] == null){                curr.next[s.charAt(i)- 'a'] = new Node();            }            if(isPal(s ,0, i)){                curr.list.add(index);            }            curr = curr.next[s.charAt(i)- 'a'];        }        curr.list.add(index);        curr.index = index;    }        public void search(String[] words, int i, List<List<Integer>> list) {        Node curr = root;        for (int j = 0; j < words[i].length(); j++) {               if (curr.index >= 0 && curr.index != i && isPal(words[i], j, words[i].length() - 1)) {                list.add(Arrays.asList(i, curr.index));            }                curr = curr.next[words[i].charAt(j) - 'a'];            if (curr == null) return;        }            for (int j : curr.list) {            if (i == j) continue;            list.add(Arrays.asList(i, j));        }    }        public boolean isPal(String word, int i, int j){        while (i < j) {            if (word.charAt(i++) != word.charAt(j--)) return false;        }            return true;        }    public List<List<Integer>> palindromePairs(String[] words) {        List<List<Integer>> res = new ArrayList<>();            for (int i = 0; i < words.length; i++) {            insert(words[i],i);        }            for (int i = 0; i < words.length; i++) {            search(words, i, res);        }            return res;    }}class Node{    int index;    List<Integer> list;    Node[] next;        Node(){        index = -1;        next = new Node[26];        list = new ArrayList<Integer>();    }}