<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:
    bool palin(string str){
        int n = str.size();
        int i=0,j=n-1;
        while(i<j)if(str[i++]!=str[j--])return 0;
        return 1;
    }
    vector<vector<int>> palindromePairs(vector<string>& words) {
        vector<vector<int>> ans;
        map<string,int> mp;
        int n = words.size();
        for(int i=0;i<n;i++){
            string str = words[i];
            reverse(str.begin(),str.end());
            mp[str]=i;
        }
        if(mp.find("")!=mp.end()){
            for(int i=0;i<n;i++){
                if(mp[""]==i)continue;
                if(palin(words[i]))ans.push_back({i,mp[""]});
            }
        }
        for(int i=0;i<n;i++){
            string str = words[i] , right = words[i] , left = "";
            for(int j=0;j<str.size();j++){
                left += str[j];
                right.erase(right.begin());
                if(mp.find(left)!=mp.end() && palin(right) && mp[left]!=i){
                    ans.push_back({i,mp[left]});
                }
                if(mp.find(right)!=mp.end() && palin(left) && mp[right]!=i){
                    ans.push_back({mp[right],i});
                }
            }
        }
        return ans;
    }
};
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