<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, data=None):
        self.data = data
        self.children = {}
        self.index = -1

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

    def insert(self, idx, word):
        curr = self.root
        for ch in list(word):
            if ch not in curr.children:
                curr.children[ch] = TrieNode(ch)
            curr = curr.children[ch]
        curr.index = idx

    def isPalindrome(self, s, l, h):
        while l <= h:
            if s[l] != s[h]:
                return False
            l += 1
            h -= 1
        return True

    def searchCase2(self, curr, temp, s):
        if curr.index != -1:
            if self.isPalindrome(s, 0, len(s)-1):
                temp.append(curr.index)

        for ch, child in curr.children.items():
            self.searchCase2(child, temp, s+ch)

    def search(self, word, temp):
        curr = self.root
        
        # case 1: when a prefix of string word, exactly matched with a word in trie.
        for i in range(len(word)):
            ch = word[i]
            if curr.index != -1:
                # check rest of the word is palindrome.
                if self.isPalindrome(word, i, len(word)-1):
                    temp.append(curr.index)

            if ch in curr.children:
                curr = curr.children[ch]
            else:
                return

        # case 2: search word is a prefix of a word in the trie.
        # check remaining subtrees in the trie for the palindrome.

        self.searchCase2(curr, temp, "")


    def palindromePairs(self, words: List[str]) -> List[List[int]]:
        
        # insert all words its reversed order to check the palindrome.
        for i in range(len(words)):
            self.insert(i, words[i][::-1])

        ans = []
        # find palidromic pairs of each word.
        for i in range(len(words)):
            myPalindrome = []
            self.search(words[i], myPalindrome)

            for idx in myPalindrome:
                if idx != i:    
                    ans.append([i, idx])

        return ans
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