<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 = {}

        for i in range(len(words)):
            node = trie 
            for le in words[i][::-1]:
                if le not in node:
                    node[le] = {}
                node = node[le]
            node['end'] = i
        
        def checkpal(word):
            i = 0 
            j = len(word)-1
            while i < j:
                if word[i] != word[j]:
                    return False 
                else:
                    i += 1
                    j -= 1
            return True
            

        def backtrack(node, curr):
            result = []
            if 'end' in node:
                if checkpal(curr):
                    result.append(node['end'])
            
            for n in node:
                if n != 'end':
                    result += backtrack(node[n], curr+n)

            return result
                
        
        ans = []

        for i in range(len(words)):
            node = trie
            valid = True
            for j in range(len(words[i])):
                if 'end' in node and checkpal(words[i][j::]):
                    ans.append([i, node['end']])

                if words[i][j] not in node:
                    valid = False
                    break
                else:
                    node = node[words[i][j]]

            if valid:
                for cand in backtrack(node, ''):
                    if i != cand:
                        ans.append([i, cand])               
                    
        return ans
                
52413