<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) {
        unordered_map<string, int> wmap;
        vector<vector<int>> ans;
        for (int i = 0; i < words.size(); i++)
            wmap[words[i]] = i;
        for (int i = 0; i < words.size(); i++) {
            if (words[i] == "") {
                for (int j = 0; j < words.size(); j++) {
                    string& w = words[j];
                    if (isPal(w, 0, w.size()-1) && j != i) {
                        ans.push_back(vector<int> {i, j});
                        ans.push_back(vector<int> {j, i});
                    }
                }
                continue;
            }
            string bw = words[i];
            reverse(bw.begin(), bw.end());
            if (wmap.find(bw) != wmap.end()) {
                int res = wmap[bw];
                if (res != i) ans.push_back(vector<int> {i, res});
            }
            for (int j = 1; j < bw.size(); j++) {
                if (isPal(bw, 0, j-1)) {
                    string s = bw.substr(j, bw.size()-j);
                    if (wmap.find(s) != wmap.end())
                        ans.push_back(vector<int> {i, wmap[s]});
                }
                if (isPal(bw, j, bw.size()-1)) {
                    string s = bw.substr(0, j);
                    if (wmap.find(s) != wmap.end())
                        ans.push_back(vector<int> {wmap[s], i});
                }
            }
        }
        return ans;
    }
    
private:
    bool isPal(string& word, int i, int j) {
        while (i < j)
            if (word[i++] != word[j--]) return false;
        return true;
    }
};
486596