<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 pal(string str)
    {
        if(str.size()==1 || str.size()==0)
        {
            return true;
        }
        string rev(str);
        reverse(rev.begin(),rev.end());
        if(rev==str)return true;
        return false;
    }


   vector<vector<int>> palindromePairs(vector<string>& words) {
        vector<vector<int>> ans; // to store answer
        unordered_map<string,int> m; // unordered map is used because its search time is O(1) for avg case 
        for(int i=0;i<words.size();i++){
            string s = words[i];
            reverse(s.begin(),s.end());
            m[s] = i; // storing reverse strings in map
        }
		// if we have empty string in given words then for this we only check if words have palindrome words in it and insert that pair in ans
        if(m.find("") != m.end()) {
            for(int i = 0; i < words.size(); i++) {
                if(i == m[""]) continue;
                if(pal(words[i])){ ans.push_back({i,m[""]});} //ex {"a",""};
            }
        }
		// Now traversing words in words 
        for(int i=0;i<words.size();i++){
			// storing i'th word in r and initializing l as empty string
            string r = words[i];
            string l = "";
			// Now traversing i'th word ans poping start character from r and pushing it to l and checking 
			// if we can find that in map and also if l or r is palindrome and final check is that if found then 
			// must not have same index so after these checks we can add those index to answer
            for(int j = 0; j < words[i].length() ; j++){
                l.push_back(words[i][j]);
                r.erase(0,1);
                if(m.find(l) != m.end() && pal(r) && m[l] != i) ans.push_back({i, m[l]});
                if(m.find(r) != m.end() && pal(l) && m[r] != i) ans.push_back({m[r], i});
            }
        }
        return ans;
    }
};
100024