<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 isPal(string s) {
        int start = 0;
        int end = s.size() - 1;
        while (start < end) {
            if (s[start] == s[end]) {
                start++;
                end--;
            } else{
                return false;
            }
        }
        return true;
    }
    vector<vector<int>> palindromePairs(vector<string>& words) {
        unordered_map<string, int> map;
        for (int i = 0; i < words.size(); i++) {
            string rev = words[i];
            reverse(rev.begin(), rev.end());
            map[rev] = i;
        }

        vector<vector<int>> res;
        for (int i = 0; i < words.size(); i++) {
            string cur = words[i];
            for (int j = 0; j <= cur.size(); j++) {
                string left = cur.substr(0, j);
                string right = cur.substr(j, cur.size() - j);
                if (!left.empty() && isPal(left) && map.find(right) != map.end() && map[right] != i) {
                    res.push_back({map[right], i});
                }
                if (map.find(left) != map.end() && isPal(right) && map[left] != i) {
                    res.push_back({i, map[left]});
                }
            }
        }
        return res;
    }
};
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