<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 = {}
        self.end = -1
        self.palin_suffix = []
        
class Solution:
    def palindromePairs(self, words: List[str]) -> List[List[int]]:
#1. Words in the Trie that are the reverse of our current word.
#2. Words in the Trie that start with the reverse of our current word and then finish in a palindrome.
#3. Words in the Trie that are the reverse of the first part of our current word, and then what's left of our current word forms a palindrome.
        trie = TrieNode()
        # Create the Trie and add the reverses of all the words.
        for idx,word in enumerate(words):
            word = word[::-1]
            cur = trie
            for j, char in enumerate(word):
                if word[j:] == word[j:][::-1]:
                    cur.palin_suffix.append(idx)
                if char not in cur.children:
                    cur.children[char] = TrieNode()
                cur = cur.children[char]
            cur.end = idx
        # Look up each word in the Trie and find palindrome pairs.
        res = []
        for idx, word in enumerate(words):
            cur = trie
            completed = True
            for j, char in enumerate(word):
                #Case 3
                if cur.end != -1 and word[j:] == word[j:][::-1]:
                    res.append([idx,cur.end])  
                if char not in cur.children:
                    completed = False
                    break
                cur = cur.children[char]
            if completed:
                #Case 1
                if cur.end != -1 and cur.end != idx:
                    res.append([idx,cur.end])
                #Case 2
                for j in cur.palin_suffix:
                    res.append([idx,j])    
        return res
                

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