<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 {
    struct Node{
        map<char, Node*> cont;
        int idx;
        Node() : idx(-1) {}
    };
    private:
    Node* root = new Node();
    void add_word(string s, int idx){
        Node* node = root;
        int ni = s.size()-1;
        while(ni >= 0){
            if(node->cont.count(s[ni]) == 0){
                Node* new_node = new Node();
                node->cont[s[ni]] = new_node;
            }
            node = node->cont[s[ni]];
            ni--;
        }
        node->idx = idx;
    }
    void find_pal(string s, int idx, vector<vector<int>>& found){
        Node* node = root;
        int ni = 0;
        while(ni < s.size()){
            if(node->idx > -1 && is_palindrome(s.substr(ni))) found.push_back({idx, node->idx});
            if(node->cont.count(s[ni]) == 0) return;
            node = node->cont[s[ni++]];
        }
        queue<pair<Node*, string>> my_q;
        my_q.push({node, ""});
        while(!my_q.empty()){
            pair<Node*, string> cur_p = my_q.front();
            my_q.pop();
            if(cur_p.first->idx > -1 && is_palindrome(cur_p.second) && cur_p.first->idx != idx) 
                found.push_back({idx, cur_p.first->idx});
            for(auto& x: cur_p.first->cont){
                if(x.second){
                    my_q.push({x.second, cur_p.second + x.first});
                }
            }
        }
        return;
    }
    bool is_palindrome(string s){
        if(s.size() < 2) return true;
        int l = 0, r = s.size()-1;
        while(l < r){
            if(s[l] != s[r]) return false;
            l++;
            r--;
        }
        return true;
    }
public:
    vector<vector<int>> palindromePairs(vector<string>& words) {
        vector<vector<int>> found;
        for(int ni = 0; ni < words.size(); ni++){
            add_word(words[ni], ni);
        }
        for(int ni = 0; ni < words.size(); ni++){
            find_pal(words[ni], ni, found);
        }
        return found;
    }
};
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