<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.children = defaultdict(TrieNode)
        self.is_end = False
        self.index = -1  
    
    def addWord(self, word, index):
        node = self
        for char in word:
            if char not in node.children: 
                node.children[char] = TrieNode()
            node = node.children[char]
        node.is_end = True
        node.index = index

class Solution:
    def palindromePairs(self, words: List[str]) -> List[List[int]]:
        # Trie data structure. 
        # Build the Trie. 
        result, root = [], TrieNode()
        for index, word in enumerate(words):
            root.addWord(word[::-1], index)
        
        # Find results/ 
        for index, word in enumerate(words):
            node = root 
            for i, char in enumerate(word): 
                if node.is_end and self.isPalindrome(word[i:]):
                    result.append([index, node.index])
                if char not in node.children:
                    break
                node = node.children[char]
            # If for loop executes without breaking. 
            else:
                self.check_remaining_suffixes(node, index, "", result)
         
        return result
    
    def check_remaining_suffixes(self, node, index, suffix, result):
        if node.is_end and index != node.index and self.isPalindrome(suffix):
            result.append([index, node.index])
        
        for char, child in node.children.items():
            self.check_remaining_suffixes(child, index, suffix + char, result)

    def isPalindrome(self, word):
        return word == word[::-1] 
    




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