<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 {
public:
    TrieNode* children[26];
    int wordIndex;
    TrieNode() {
        for (int i = 0; i < 26; i++) {
            children[i] = nullptr;
        }
        wordIndex = -1;
    }
};

class Trie {
public:
    TrieNode* root;
    Trie() {
        root = new TrieNode();
    }

    void insert(const string& word, int index) {
        TrieNode* curr = root;
        for (int i = word.length() - 1; i >= 0; i--) {
            int ch = word[i] - 'a';
            if (!curr->children[ch]) {
                curr->children[ch] = new TrieNode();
            }
            curr = curr->children[ch];
        }
        curr->wordIndex = index;
    }

    vector<int> getIndices(TrieNode* curr) {
        vector<int> indices;
        queue<TrieNode*> bfs;
        bfs.push(curr);
        while (!bfs.empty()) {
            TrieNode* temp = bfs.front();
            bfs.pop();
            if (temp->wordIndex != -1) {
                indices.push_back(temp->wordIndex);
            }
            for (int i = 0; i < 26; i++) {
                if (temp->children[i]) {
                    bfs.push(temp->children[i]);
                }
            }
        }
        return indices;
    }
};

class Solution {
public:
    bool isPalindrome(const string& word, int si, int ei) {
        while (si < ei) {
            if (word[si] != word[ei]) return false;
            si++;
            ei--;
        }
        return true;
    }

    vector<vector<int>> palindromePairs(vector<string>& words) {
        Trie* trie = new Trie();
        int sz = words.size();
        vector<vector<int>> result;
        for (int i = 0; i < sz; i++) {
            trie->insert(words[i], i);
        }

        for (int i = 0; i < sz; i++) {
            string word = words[i];
            TrieNode* curr = trie->root;
            int j = 0;
            for (; j < word.length() && curr; j++) {
                int ch = word[j] - 'a';
                if (curr->wordIndex != -1 && curr->wordIndex != i && isPalindrome(word, j, word.length() - 1)) {
                    result.push_back({i, curr->wordIndex});
                }
                curr = curr->children[ch];
            }
            if (curr) {
                vector<int> indices = trie->getIndices(curr);
                for (int k : indices) {
                    if (k != i && isPalindrome(words[k], 0, words[k].length() - j- 1)) {
                        result.push_back({i, k});
                    }
                }
            }
        }

        delete trie;
        return result;
    }
};

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