<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 Solution:
    def palindromePairs(self, words: List[str]) -> List[List[int]]:
        trie = Trie()
        for index, word in enumerate(words):
            trie.insertWord(word[::-1], index)

        ans = []
        for index, word in enumerate(words):
            node = trie.root

            charIndex = 0
            while charIndex < len(word):
                char = word[charIndex]
                remainingStr = word[charIndex:]
                if (
                    node.isEnd
                    and node.index != index
                    and remainingStr == remainingStr[::-1]
                ):
                    ans.append([index, node.index])
                if char not in node.children:
                    break
                node = node.children[char]
                charIndex += 1
            # print(index, word, charIndex, node.index)

            if charIndex == len(word):

                def dfs(node, currStr, ans, index):

                    if node.isEnd and node.index != index and currStr == currStr[::-1]:
                        ans.append([index, node.index])

                    if not node.children:
                        return

                    for child in node.children:
                        dfs(node.children[child], currStr + child, ans, index)

                dfs(node, "", ans, index)

            if (
                "" in trie.root.children
                and trie.root.children[""].index != index
                and word == word[::-1]
            ):
                ans.append([index, trie.root.children[""].index])

        return ans


class TrieNode:
    def __init__(self):
        self.children = {}
        self.isEnd = False
        self.index = -1


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

    def insertWord(self, word, index):
        node = self.root

        if word == "":
            node.children[word] = TrieNode()
            node = node.children[word]

        for char in word:
            if not char in node.children:
                node.children[char] = TrieNode()
            node = node.children[char]
        node.isEnd = True
        node.index = index

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