<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 {

    unordered_map<string, int> m;
public:
    vector<vector<int>> palindromePairs(vector<string>& words) {

        for (int i=0; i<words.size(); i++) {
            m[words[i]] = i;
        }

        set<vector<int>> ansSet;

        for (int i=0; i<words.size(); i++) {
            if (words[i].size()==0) {
                continue;
            }
            string rev = "";
            int end = words[i].size()-1;
            for (int j=0; j<=end; j++) {
                if (isPalindrome(words[i], j, end)) {
                    if (m.count(rev)!=0 && m[rev]!=i) {
                        ansSet.insert({i, m[rev]});
                                                // cout << "pusing : " << m[rev]  << " " << rev << " "<< i << endl;
                    }
                }
                rev = words[i][j] + rev;
            }
            if (m.count(rev)!=0 && m[rev]!=i) {
            // cout << "rev: " << rev << i << " " << m[rev]<< endl;
                ansSet.insert({i, m[rev]});
            }

            rev = "";
            // rev += words[i].back();
            for (int j=words[i].size()-1; j>=0; j--) {
                if (isPalindrome(words[i], 0, j)) {
                    if (m.count(rev)!=0 && m[rev]!=i) {
                        // cout << "pusing : " << m[rev]  << " " << rev << " "<< i << endl;
                        ansSet.insert({m[rev], i});
                    }
                }
                rev += words[i][j];
            }
            if (m.count(rev)!=0 && m[rev]!=i) {
                ansSet.insert({m[rev], i});
            }
        }
        vector<vector<int>> ans;
        ans.assign(ansSet.begin(), ansSet.end());
        return ans;
    }



    bool isPalindrome(string& s, int st, int end) {
        // cout << "pal : " << s  << " " << st << " "<< end << endl; 
        while(st<=end && s[st] == s[end]) {
            st++;
            end--;
        }
        bool palCheck =  st>end? true : false;
        // cout << palCheck << endl;
        return palCheck;
    }
};
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