<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>

struct Node {
    Node* links[26];
    int wordIndex = -1; // Store index of word that ends here
    vector<int> palindromesBelow; // Store indices of words that are palindrome from this point to end

    bool containsKey(char ch) {
        return links[ch - 'a'] != NULL;
    }

    void put(char ch, Node* node) {
        links[ch - 'a'] = node;
    }

    Node* get(char ch) {
        return links[ch - 'a'];
    }

    void setEnd(int index) {
        wordIndex = index;
    }

    bool isEnd() {
        return wordIndex != -1;
    }
};

class Trie {
private:
    Node* root;

    bool isPalindrome(string& word, int left, int right) {
        while (left < right) {
            if (word[left++] != word[right--]) return false;
        }
        return true;
    }

public:
    Trie() {
        root = new Node();
    }

    void insert(string word, int index) {
        Node* node = root;
        for (int i = word.length() - 1; i >= 0; i--) {
            if (isPalindrome(word, 0, i)) {
                node->palindromesBelow.push_back(index);
            }
            if (!node->containsKey(word[i])) {
                node->put(word[i], new Node());
            }
            node = node->get(word[i]);
        }
        node->setEnd(index);
        node->palindromesBelow.push_back(index);
    }

    void search(string word, int index, vector<vector<int>>& result) {
        Node* node = root;
        for (int i = 0; i < word.length(); i++) {
            if (node->isEnd() && isPalindrome(word, i, word.length() - 1)) {
                result.push_back({index, node->wordIndex});
            }
            if (!node->containsKey(word[i])) {
                return;
            }
            node = node->get(word[i]);
        }

        for (int palindromeIndex : node->palindromesBelow) {
            if (palindromeIndex != index) {
                result.push_back({index, palindromeIndex});
            }
        }
    }
};

class Solution {
public:
    vector<vector<int>> palindromePairs(vector<string>& words) {
        Trie trie;
        vector<vector<int>> result;

        // Insert all words into the Trie in reverse order
        for (int i = 0; i < words.size(); i++) {
            trie.insert(words[i], i);
        }

        // Search for palindrome pairs
        for (int i = 0; i < words.size(); i++) {
            trie.search(words[i], i, result);
        }

        return result;
    }
};
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