<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 Manacher {
public:
    vector<int> p;
    void run_manacher(const string& s) {
        int n = s.size();
        p.assign(n, 0);
        int l = 0, r = 0;
        for (int i = 1; i < n; i++) {
            if (i < r)
                p[i] = min(r - i, p[l + r - i]);
            while (i - p[i] >= 0 && i + p[i] < n && s[i - p[i]] == s[i + p[i]])
                p[i]++;
            if (i + p[i] > r) {
                l = i - p[i];
                r = i + p[i];
            }
        }
    }

    Manacher(const string& s) {
        string t = "*";
        for (char c : s) {
            t += c;
            t += '*';
        }
        run_manacher(t);
    }

    int getLongest(int cen, int odd) {
        int pos = 2 * cen + 1 + (!odd);
        return p[pos] - 1;
    }

    bool checkPalindrome(int l, int r) {
        return (r - l + 1) <= getLongest((l + r) / 2, l % 2 == r % 2);
    }
};
class Solution {
public:
    vector<vector<int>> palindromePairs(vector<string>& words) {
        int n=words.size();
        unordered_map<string,int>m1;
        unordered_map<string,int>m2;
        for(int i=0;i<n;i++) m1[words[i]]=i;
        for(int i=0;i<n;i++){
            string d=words[i];
            reverse(d.begin(),d.end());
            m2[d]=i;
        }        
        set<vector<int>>ans;
        int cnt=0;
        for(auto &word:words){
            string s="";
            Manacher k(word);
            for(int i=0;i<word.size();i++){
                if(k.checkPalindrome(i,word.size()-1) and m2.count(s) and m2[s]!=cnt){
                    ans.insert({cnt,m2[s]});        
                }
                s+=word[i];
            }
            if(m2.count(s) and m2[s]!=cnt) ans.insert({cnt,m2[s]});
            cnt++;
        }
        cnt=0;
        for(auto &word:words){
            reverse(word.begin(),word.end());
            string s="";
            Manacher k(word);
            for(int i=0;i<word.size();i++){
                if(k.checkPalindrome(i,word.size()-1) and m1.count(s) and m1[s]!=cnt){
                    ans.insert({m1[s],cnt});        
                }
                s+=word[i];
            }
            if(m1.count(s) and m1[s]!=cnt) ans.insert({m1[s],cnt});
            cnt++;
        }
        vector<vector<int>>res(ans.begin(),ans.end());
        return res;
    }
};
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