<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 {
public:
    vector<vector<int>> palindromePairs(vector<string>& words) {
        int n = words.size();
        set<vector<int>> st;
        function<void(vector<string>&, int)> op = [&](vector<string> &words, int flip)
        {
            unordered_map<string, int> mp;
            function<int(string&)> check = [&](string &s)
            {
                int i = 0, j = s.size() - 1;
                while(i < j)
                {
                    if(s[i] != s[j]) return false;
                    i++, j--;
                }
                return true;
            };
            vector<int> vis(n);
            for(int i = 0; i < n; i++)
            {
                reverse(words[i].begin(), words[i].end());
                mp[words[i]] = i;
                reverse(words[i].begin(), words[i].end());
                vis[i] = check(words[i]);
            }
            for(int i = 0; i < n; i++) 
            {
                if(words[i] != "") continue;
                for(int j = 0; j < n; j++)
                {
                    if(i == j || !vis[j]) continue;
                    st.insert({i, j});
                    st.insert({j, i});
                }
            }
            for(int i = 0; i < n; i++)
            {
                string s = "", t = words[i];
                reverse(t.begin(), t.end());
                for(int j = 0; j < words[i].size(); j++)
                {
                    s += words[i][j];
                    t.pop_back();
                    if(mp.find(s) != mp.end() && mp[s] != i && check(t)) 
                    {
                        if(!flip) st.insert({i, mp[s]});
                        else st.insert({mp[s], i});
                    }
                }
            }
        };
        op(words, 0);
        for(int i = 0; i < n; i++) reverse(words[i].begin(), words[i].end());
        op(words, 1);
        vector<vector<int>> ans;
        for(auto it : st) ans.push_back(it);
        return ans;
    }
};
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