<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 isPallindrome(string s,int i,int j){
        while(i<j){
            if(s[i++]!=s[j--]) return false;
        }
        return true;
    }

    vector<vector<int>> palindromePairs(vector<string>& words) {
        int n=words.size();
        unordered_map<string,int>mp;
        vector<vector<int>>res;
        for(int i=0;i<n;i++){
            string rev=words[i];
            reverse(rev.begin(),rev.end());
            mp[rev]=i;
        }
        for(int i=0;i<n;i++){
            string currWord=words[i];
            int len=currWord.size();
            if(mp.find("")!=mp.end()&&isPallindrome(currWord,0,len-1)&&mp[""]!=i){
                res.push_back({i,mp[""]});
                res.push_back({mp[""],i});
            }
            auto it=mp.find(currWord);
            if(it!=mp.end()&&it->second!=i){
                res.push_back({i,it->second});
            }
            for(int j=1;j<len;j++){
                if(isPallindrome(currWord,0,j-1) && mp.count(currWord.substr(j))) {
                    res.push_back({mp[currWord.substr(j)], i});
                }
                if(isPallindrome(currWord,j,len-1)&&mp.count(currWord.substr(0,j))) {
                    res.push_back({i,mp[currWord.substr(0,j)]});
                }
            }
        }
        return res;
    }
};
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