<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: 
    int ending = -1; 
    unordered_map<char, TrieNode*> mp; 
    vector<int> palindromePrefixRemaining; 
    TrieNode() {}; 
}; 
class Solution {
public: 
    TrieNode* root; 
    bool hasPalindromeRemaining(string& s, int i) {
        int l = i, r = s.size() - 1; 
        while (l < r) {
            if (s[l++] != s[r--]) return false; 
        }
        return true; 
    }
    vector<vector<int>> palindromePairs(vector<string>& words) {
        root = new TrieNode(); 
        for (int i = 0; i < words.size(); i++) {
            TrieNode* cur = root; 
            string word = words[i]; 
            string rev_w = words[i]; 
            reverse(rev_w.begin(), rev_w.end()); 
            for (int j = 0; j < rev_w.size(); j++) {
                if (hasPalindromeRemaining(rev_w, j)) {
                    cur->palindromePrefixRemaining.push_back(i); 
                }
                char c = rev_w[j]; 
                if (!cur->mp.count(c)) {
                    cur->mp[c] = new TrieNode(); 
                }
                cur = cur->mp[c];  
            }
            cur->ending = i; 
        }
        vector<vector<int>> pairs; 
        for (int i = 0; i < words.size(); i++) {
            string word = words[i]; 
            TrieNode* cur = root; 
            for (int j = 0; j < word.size(); j++) {
                if (cur->ending != -1 && hasPalindromeRemaining(word, j)) {
                    pairs.emplace_back(vector<int>{i, cur->ending}); 
                }
                char c = word[j]; 
                if (!cur->mp.count(c)) {
                    cur = nullptr; break; 
                }
                cur = cur->mp[c]; 
            }
            if (cur == nullptr) continue; 
            if (cur->ending != -1 && cur->ending != i) {
                pairs.emplace_back(vector<int>{i, cur->ending}); 
            }
            for (int other : cur->palindromePrefixRemaining) {
                pairs.emplace_back(vector<int>{i, other}); 
            }
        }
        return pairs; 
    }
};
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