<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 Trie {
    unordered_map<char,Trie*> M;
    vector<int> PAL;
    vector<int> END;
};

class Solution {
public:
    vector<vector<int>> palindromePairs(vector<string>& words) {
        Trie* root = new Trie();
        for (int i = 0; i < words.size(); i++) {
            addTree(root, words[i], i);
        }
        vector<vector<int>> res;
        for (int i = 0; i < words.size(); i++) {
            Trie* t = root;
            int j = words[i].size() - 1;
            for (; j >= 0; j--) {
                if (isPalindrome(words[i], 0, j)) {
                    for (auto& idx : t->END) {
                        res.push_back({idx, i});
                    }
                }
                if (!t->M.count(words[i][j])) {
                    break;
                }
                t = t->M[words[i][j]];
            }
            if (j == -1) {
                for (auto& idx : t->PAL) {
                    if (idx != i) {
                        res.push_back({idx, i});
                    }
                }
            }
        }
        return res;
    }

    void addTree(Trie* root, const string& s, int idx) {
        for (int i = 0; i < s.length(); i++) {
            if (isPalindrome(s, i, s.length() - 1)) {
                root->PAL.push_back(idx);
            }
            if (!root->M.count(s[i])) {
                root->M[s[i]] = new Trie();
            }
            root = root->M[s[i]];
        }
        root->END.push_back(idx);
        root->PAL.push_back(idx);
    }

    bool isPalindrome(const string& s, int l, int r) {
        for (int i = 0; i < (r - l + 1) / 2; i++) {
            if (s[l + i] != s[r - i]) {
                return false;
            }
        }
        return true;
    }
};
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