<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) {
        vector<vector<int>> ans;
        unordered_map<string,int> m;
        set<int>s;
        int len = words.size();
        for(int i=0;i<len;i++){
            m[words[i]]=i;
            s.insert(words[i].size());
        }
        for(int i=0;i<len;i++){
            string tmp = words[i];
            int tmp_len = tmp.size();
            reverse(tmp.begin(),tmp.end());
            if(m.count(tmp) && m[tmp]!=i){
                ans.push_back({m[tmp],i});
            }

           auto a = s.find(tmp_len);
            for(auto it = s.begin(); it != a; ++it) {
                int cur_len = *it;
              
                if(isPalindrome(tmp, 0, tmp_len-cur_len-1) && m.count(tmp.substr(tmp_len-cur_len))) {
                    ans.push_back({i , m[tmp.substr(tmp_len-cur_len)]});
                }
                
                if(isPalindrome(tmp, cur_len, tmp_len-1) && m.count(tmp.substr(0, cur_len))) {
                    ans.push_back({m[tmp.substr(0, cur_len)], i});
                }
            }
            
        }
        
        return ans;
    }
    
    bool isPalindrome(string &s, int left, int right) {
        while(left < right) {
            if(s[left++] != s[right--]) return false;
        }
        
        return true;
    }
};


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