<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>> res;
        std::unordered_map<string, int> _map;
        std::set<int> lens;
        for (int i = 0; i < words.size(); ++i) {
            _map[words[i]] = i;
            lens.insert(words[i].size());
        }

        for (int i = 0; i < words.size(); ++i) {
            string t = words[i];
            std::reverse(t.begin(), t.end());

            if (_map.find(t) != _map.end() && _map[t] != i)
                res.push_back({i, _map[t]});

            auto it = lens.find(t.size());
            for (auto iter = lens.begin(); iter != it; ++iter) {
                int len = *iter;
                // abXXX + ba could be palidrom if XXX is pal and ba exists,
                // note t is revesed so XXX ba
                string sub = t.substr(t.size() - len, len);
                if (isPal(t, 0, t.size() - len - 1) && _map.count(sub))
                    res.push_back({i, _map[sub]});
                //ba + XXXab could be pal if XXX is pal and ba exist
                // t is reversed so baXXX 
                sub = t.substr(0, len);
                if (isPal(t, len, t.size()-1) && _map.count(sub))
                res.push_back({_map[sub], i});
            }
        }
        return res;
    }

    bool isPal(const string& s, int l, int r) {
        assert(l >= 0 && r < s.size());
        while (l < r) {
            if (s[l++] != s[r--])
                return false;
        }
        return true;
    }
};
234484