<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):
        self.w = -1
        self.p = []
        self.children = collections.defaultdict(TrieNode)

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

    @staticmethod
    def is_palindrome(word: str) -> bool :
        return word[::] == word[::-1]

    def insert(self, index, word: str) -> None:
        node = self.root
        for i, char in enumerate(word[::-1]) : 
            if self.is_palindrome(word[0:len(word) - i]) :
                node.p.append(index)
            node = node.children[char]
        node.w = index

    def search(self, index, word: str) -> List[List[int]]:
        result = []
        node = self.root

        while word : 
            if node.w >= 0 :
                if self.is_palindrome(word) : 
                    result.append([index, node.w])
            if not word[0] in node.children : 
                return result
            node = node.children[word[0]]
            word = word[1:]

        if node.w >= 0 and node.w != index :
            result.append([index, node.w])
        
        for p in node.p : 
            result.append([index, p])
        
        return result
        

class Solution:
    def palindromePairs(self, words: List[str]) -> List[List[int]]:
        
        trie = Trie()
        
        for i, word in enumerate(words) : 
            trie.insert(i, word)

        results = []

        for i, word in enumerate(words) : 
            results.extend(trie.search(i, word))

        return results
            

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