class Solution {
    public List<List<Integer>> palindromePairs(String[] words) {
        TrieNode trie = new TrieNode();
        for (int i = 0; i < words.length; i++) trie.add(words[i], i);

        List<List<Integer>> result = new ArrayList<List<Integer>>();

        for (int i = 0; i < words.length; i++) {
            TrieNode n = trie.findBackwards(words[i]);
            if (n != null && n.wordIndex != -1 && n.wordIndex != i) {
                ArrayList<Integer> r = new ArrayList<>();
                r.add(n.wordIndex);
                r.add(i);
                result.add(r);
            }
        }

        return result;
    }

    class TrieNode {
        int wordIndex;
        Map<Character, TrieNode> next;

        public TrieNode() {
            next = new HashMap<Character, TrieNode>();
            wordIndex = -1;
        }

        public void add(String s, int wordIndex) {
            addInt(s, 0, wordIndex);
        }

        private void addInt(String s, int index, int wordIndex) {
            if (index >= s.length()) {
                this.wordIndex = wordIndex;
            }
            else {
                char c = s.charAt(index);
                TrieNode n = getOrAdd(c);
                n.addInt(s, index +  1, wordIndex);
            }
        }

        private TrieNode getOrAdd(char c) {
            TrieNode n = next.get(c);
            if (n == null) {
                n = new TrieNode();
                next.put(c, n);
            }
            return n;
        }

        public TrieNode findBackwards(String s) {
            return findBackwards(s, s.length() - 1);
        }

        private TrieNode findBackwards(String s, int idx) {
            if (idx < 0) return this;

            char c = s.charAt(idx);
            TrieNode n = next.get(c);
            if (n == null) return null;

            return n.findBackwards(s, idx - 1);
        }
    }
}