<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>

def is_palindrome(word, substr_start=0, substr_end=float("inf")):
    l, r = substr_start, min(substr_end, len(word)-1)
    while l < r:
        if word[l] != word[r]:
            return False
        l += 1
        r -= 1
    return True

class TrieNode:
    def __init__(self):
        self.next = collections.defaultdict(TrieNode)
        self.word_index = None
        self.palindrome_suffixes = []
    
    def insert(self, word, index):
        node = self
        for i in range(len(word)-1, -1, -1):
            if is_palindrome(word, 0, i):
                node.palindrome_suffixes.append(index)
            node = node.next[word[i]]
        node.word_index = index
    
    def search(self, word, start=0):
        node = self
        for i in range(start, len(word)):
            if word[i] not in node.next:
                return None
            node = node.next[word[i]]
        return node

class Solution:
    def palindromePairs(self, words: List[str]) -> List[List[int]]:
        trie = TrieNode()
        for i, word in enumerate(words):
            trie.insert(word, i)
        res = []
        for i, word in enumerate(words):
            node = trie.search(word)
            if node is not None:
                # reverse of word in trie
                if node.word_index is not None:
                    if node.word_index != i:
                        res.append([i, node.word_index])
        
                # reverse of word in trie but as a suffix attached to a palindrome prefix
                for suffix in node.palindrome_suffixes:
                    res.append([i, suffix])
                
            # prefix of word in trie; rest of word is palindrome suffix
            j = 0
            node = trie
            while j <= len(word):
                if j < len(word) and node.word_index is not None and is_palindrome(word, j):
                    res.append([i, node.word_index])
                if j == len(word):
                    break
                if word[j] not in node.next:
                    break
                node = node.next[word[j]]
                j += 1
        return res
231836