<p>You are given a <strong>0-indexed</strong> array of <strong>unique</strong> strings <code>words</code>.</p>

<p>A <strong>palindrome pair</strong> is a pair of integers <code>(i, j)</code> such that:</p>

<ul>
	<li><code>0 &lt;= i, j &lt; words.length</code>,</li>
	<li><code>i != j</code>, and</li>
	<li><code>words[i] + words[j]</code> (the concatenation of the two strings) is a <span data-keyword="palindrome-string">palindrome</span>.</li>
</ul>

<p>Return <em>an array of all the <strong>palindrome pairs</strong> of </em><code>words</code>.</p>

<p>You must write an algorithm with&nbsp;<code>O(sum of words[i].length)</code>&nbsp;runtime complexity.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;abcd&quot;,&quot;dcba&quot;,&quot;lls&quot;,&quot;s&quot;,&quot;sssll&quot;]
<strong>Output:</strong> [[0,1],[1,0],[3,2],[2,4]]
<strong>Explanation:</strong> The palindromes are [&quot;abcddcba&quot;,&quot;dcbaabcd&quot;,&quot;slls&quot;,&quot;llssssll&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;bat&quot;,&quot;tab&quot;,&quot;cat&quot;]
<strong>Output:</strong> [[0,1],[1,0]]
<strong>Explanation:</strong> The palindromes are [&quot;battab&quot;,&quot;tabbat&quot;]
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> words = [&quot;a&quot;,&quot;&quot;]
<strong>Output:</strong> [[0,1],[1,0]]
<strong>Explanation:</strong> The palindromes are [&quot;a&quot;,&quot;a&quot;]
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= words.length &lt;= 5000</code></li>
	<li><code>0 &lt;= words[i].length &lt;= 300</code></li>
	<li><code>words[i]</code> consists of lowercase English letters.</li>
</ul>

class TrieNode:
    def __init__(self, is_word=False, word_index=-1):
        self.is_word = is_word
        self.word_index = word_index
        self.children = {}

class Trie:

    def __init__(self):
        self.root = TrieNode()

    def insert(self, word: str, index: int) -> None:
        cur_node = self.root

        for i in range(len(word)):
            if word[i] not in cur_node.children:
                cur_node.children[word[i]] = TrieNode()
            cur_node = cur_node.children[word[i]]
        
        cur_node.word_index = index
        cur_node.is_word = True

    # def search(self, word: str) -> Optional[TrieNode]:
    #     cur_node = self.root

    #     for i in range(len(word)):
    #         if word[i] not in cur_node.children:
    #             return None
    #         cur_node = cur_node.children[word[i]]
        
    #     if cur_node and cur_node.is_word:
    #         return cur_node
    #     return None

    # def startsWith(self, prefix: str) -> Optional[TrieNode]:
    #     cur_node = self.root

    #     for i in range(len(prefix)):
    #         if word[i] not in cur_node.children:
    #             return None
    #         cur_node = cur_node.children[word[i]]
        
    #     return cur_node

class Solution:
    def isSameReverse(self, word: str) -> bool:
        return word == word[::-1]

    def dfs(self, cur_node: TrieNode, index: int, cur_word: str) -> List[List[int]]:
        pairs = []

        if cur_node.is_word and index != cur_node.word_index and self.isSameReverse(cur_word):
            pairs.append([index, cur_node.word_index])

        for key in cur_node.children.keys():
            pairs += self.dfs(cur_node.children[key], index, cur_word + key)
        
        return pairs

    def findPairs(self, trie: Trie, word: str, index: int) -> List[List[int]]:
        pairs = []
        cur_node = trie.root

        for i in range(len(word)):
            if cur_node.is_word and index != cur_node.word_index and self.isSameReverse(word[i:]):
                pairs.append([index, cur_node.word_index])
            if word[i] not in cur_node.children:
                return pairs
            cur_node = cur_node.children[word[i]]

        pairs += self.dfs(cur_node, index, '')
        return pairs

    def palindromePairs(self, words: List[str]) -> List[List[int]]:
        trie = Trie()
        pairs = []

        for i, word in enumerate(words):
            trie.insert(word[::-1], i)
        
        for i, word in enumerate(words):
            pairs += self.findPairs(trie, word, i)

        return pairs

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