<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 {

private:
	bool checkPalindrome(string& word, int l, int r) {
		while (l < r)
		{
			if (word[l] != word[r]) return false;
			l++; r--;
		}
		return true;
	}
public:
	vector<vector<int>> palindromePairs(vector<string>& words) {
		unordered_map<string, int> um;
		vector<vector<int>> ans;
		int emptyindex = -1;
		vector<int> indexOfPalindromes;
		for (int i = 0; i < words.size(); ++i)
		{
			if (words[i] == "")
			{
				emptyindex = i;
				continue;
			}
			if (checkPalindrome(words[i], 0, words[i].size() - 1))
				indexOfPalindromes.push_back(i);
			string str = words[i];
			
			reverse(begin(str), end(str));
			um[str] = i;
		}
		for (int i = 0; i < words.size(); ++i)
		{
			for (int cut = 0; cut < words[i].size(); ++cut)
			{
			
				if (checkPalindrome(words[i], cut, words[i].size() - 1))
				{
					string right = words[i].substr(0, cut);
					if (um.find(right) != end(um) && um[right] != i)
					{
						ans.push_back(vector<int> {i, um[right]});
					}
				}
				
				if (checkPalindrome(words[i], 0, cut - 1))
				{
					string left = words[i].substr(cut);
					if (um.find(left) != end(um) && um[left] != i)
					{
						ans.push_back(vector<int> {um[left], i});
					}
				}
			}
		}
		if (emptyindex != -1)
			for (int x : indexOfPalindromes)
			{
				ans.push_back(vector<int> {emptyindex, x});
				ans.push_back(vector<int> {x, emptyindex});
			}
		return ans;
	}
};
495000