<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>

from typing import List


class Trie:

    def __init__(self):
        self.trie = {}

    def insert(self, word, index):
        base_trie = self.trie
        for c in word:
            if c in base_trie:
                base_trie = base_trie[c]
            else:
                base_trie[c] = {}
                base_trie = base_trie[c]

        base_trie["index"] = index
        base_trie["is_word"] = True

    def has_word(self, word):
        base_trie = self.trie
        for c in word:
            if c in base_trie:
                base_trie = base_trie[c]
            else:
                return False, None

        return base_trie.get("is_word", False), base_trie.get("index")

    def get_prefix(self, prefix):
        base_trie = self.trie
        for c in prefix:
            if c in base_trie:
                base_trie = base_trie[c]
            else:
                return None

        return base_trie


class Solution:
    def palindromePairs(self, words: List[str]) -> List[List[int]]:
        trie = Trie()
        # insert every word into trie
        for k, word in enumerate(words):
            trie.insert(word, k)

        ret = []
        for k, word in enumerate(words):
            if word == "":
                continue

            # exact match of reverse
            # i.e abcd -> dbca
            word_rev = word[::-1]
            has_word, idx = trie.has_word(word_rev)
            if has_word and idx != k:
                ret.append([idx, k])

            word_len = len(word)
            # palindrome prefix + suffix
            for i in range(1, word_len):
                prefix = word[:i]
                suffix = word[i:]
                prefix_rev = prefix[::-1]
                if prefix == prefix_rev:
                    has_word, idx = trie.has_word(suffix[::-1])
                    if has_word:
                        # print(idx, k)
                        ret.append([idx, k])

            # prefix + palindrome suffix
            for i in range(word_len-1, 0, -1):
                suffix = word[i:]
                suffix_rev = suffix[::-1]
                prefix = word[:i]
                if suffix == suffix_rev:
                    has_word, idx = trie.has_word(prefix[::-1])
                    if has_word:
                        ret.append([k, idx])

            # word is palindrome
            if word == word_rev:
                has_word, idx = trie.has_word("")
                # print(word, k, idx)
                if has_word:
                    ret.append([k, idx])
                    ret.append([idx, k])

        return ret
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