<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) {
        int n = words.size();
        map<string, int> m;
        for (int i = 0; i < n; ++i) {
            m[words[i]] = i;
        }

        vector<vector<int>> res;
        for (int i = 0; i < n; ++i) {
            auto& word = words[i];
            // reverse(word.begin(), word.end());
            for (int j = 0; j < word.size(); ++j) {
                if (IsPalindrome(word, j)) {
                    string tail = word.substr(j + 1);
                    reverse(tail.begin(), tail.end());
                    // cout << word << ", " << tail << endl;
                    if (m.contains(tail) && i != m[tail]) {
                        // cout << "=> " << m[tail] << ", " << i << endl;
                        res.push_back({m[tail], i});
                    }
                } 
            }
            reverse(word.begin(), word.end());
            if (m.contains(word) && i != m[word]) {
                res.push_back({i, m[word]});
            }
            for (int j = 0; j < word.size(); ++j) {
                if (IsPalindrome(word, j)) {
                    string tail = word.substr(j + 1);
                    // cout << word << ",, " << tail << endl;
                    if (m.contains(tail) && i != m[tail]) {
                        // cout << "=> " << i << ", " << m[tail] << endl;
                        res.push_back({i, m[tail]});
                    }
                } 
            }            
        }
        return res;
    }
    bool IsPalindrome(const string& word, int end) {
        int left = 0;
        int right = end;
        while (left < right) {
            if (word[left] != word[right])
                return false;
            ++left;
            --right;
        }
        return true;
    }
};
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