<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>

/**
construct a trie with reversed words. ab -> ba
traverse each word in trie
if we find a word ending before end of current word, then check if rest of the
word is palindrome if searching for ab and matched a in trie so far, check if b
is palindrome if we matched the whole word, we should look for every suffix
under the ending node and check if they are palindrome

**/
class Solution {
    struct Trie {
        std::vector<std::vector<int>> trie;
        std::vector<int> index;

        Trie() {
            trie.push_back(std::vector<int>(26));
            index.push_back(-1);
        }

        void insert(const std::string& str, int idx) {
            int node = 0;
            for (char c : str) {
                if (trie[node][c - 'a'] == 0) {
                    trie[node][c - 'a'] = trie.size();
                    trie.push_back(std::vector<int>(26));
                    index.push_back(-1);
                }
                node = trie[node][c - 'a'];
            }
            index[node] = idx;
        }

        bool is_palindrome(const std::string& str, int lo, int hi) {
            while (lo < hi) {
                if (str[lo] != str[hi])
                    return false;
                ++lo;
                --hi;
            }
            return true;
        }

        void dfs(int node, std::vector<int>& result, std::string& current) {
            if (index[node] != -1 &&
                is_palindrome(current, 0, int(current.size()) - 1)) {
                result.push_back(index[node]);
            }

            for (int i = 0; i < trie[node].size(); ++i) {
                if (trie[node][i] != 0) {
                    current.push_back('a' + i);
                    dfs(trie[node][i], result, current);
                    current.pop_back();
                }
            }
        }

        std::vector<int> find_all(const std::string& str) {
            std::vector<int> result;

            int node = 0;
            for (int i = 0; i < str.size(); ++i) {
                // assume str is "ab" and we found "a" so far
                // then check if the rest of str("b") is palindrome
                if (index[node] != -1 &&
                    is_palindrome(str, i, int(str.size()) - 1)) {
                    result.push_back(index[node]);
                }

                char c = str[i];
                if (trie[node][c - 'a'] == 0)
                    return result;

                node = trie[node][c - 'a'];
            }

            // we fully matched str in trie. now search this node recursively to
            // find palindromes underneath ab ...... ba => any palindrome in
            // between will result palindrome in overall
            std::string current;
            dfs(node, result, current);
            return result;
        }
    };

public:
    vector<vector<int>> palindromePairs(vector<string>& words) {

        Trie reverse;
        for (int i = 0; i < words.size(); ++i) {
            auto word = words[i];
            std::reverse(word.begin(), word.end());
            reverse.insert(word, i);
        }

        std::vector<std::vector<int>> result;
        for (int i = 0; i < words.size(); ++i) {
            auto indexes = reverse.find_all(words[i]);
            for (int index : indexes) {
                if (index != i) {
                    result.push_back({i, index});
                }
            }
        }
        return result;
    }
};
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