<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 isPalindrome(const std::string& s, int left, int right) {
        while (left < right) {
            if (s[left++] != s[right--]) {
                return false;
            }
        }
        return true;
    }
    vector<vector<int>> palindromePairs(vector<string>& words) {
                std::unordered_map<std::string, int> wordMap;
        std::set<int> lengths;
        int n = words.size();

        for (int i = 0; i < n; ++i) {
            wordMap[words[i]] = i;
            lengths.insert(words[i].length());
        }

        std::vector<std::vector<int>> ans;

        for (int i = 0; i < n; ++i) {
            int length = words[i].length();
            std::string reverse = std::string(words[i].rbegin(), words[i].rend());

            if (wordMap.find(reverse) != wordMap.end() && wordMap[reverse] != i) {
                ans.push_back({i, wordMap[reverse]});
            }

            for (int k : lengths) {
                if (k == length) break;

                if (isPalindrome(reverse, 0, length - 1 - k)) {
                    std::string s1 = reverse.substr(length - k);
                    if (wordMap.find(s1) != wordMap.end()) {
                        ans.push_back({i, wordMap[s1]});
                    }
                }

                if (isPalindrome(reverse, k, length - 1)) {
                    std::string s2 = reverse.substr(0, k);
                    if (wordMap.find(s2) != wordMap.end()) {
                        ans.push_back({wordMap[s2], i});
                    }
                }
            }
        }
        return ans;
    }
};
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