<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 TrieNode{
public:
    int end;
    unordered_map<char, TrieNode*> child;
    TrieNode(){
        end = -1;
    }
};

class Solution {
public:
    void buildTrie(string word, int index, TrieNode* node){
        for(auto& x: word){
            if(node->child.find(x) == node->child.end()){
                node->child[x] = new TrieNode();
            }
            node = node->child[x];
        }
        node->end = index;
    }

    vector<vector<int>> palindromePairs(vector<string>& words) {
        vector<vector<int>> ans;
        unordered_map<string, int> hash;
        TrieNode* root = new TrieNode();
        int n = words.size();
        // Build up the Trie
        for(int i = 0; i < n; i++){
            string rword = words[i];
            reverse(rword.begin(), rword.end());
            buildTrie(rword, i, root);
            if(rword == ""){root->end = i;}
        }
        // Iterate through all words.
        for(int i = 0; i < n; i++){
            string word = words[i];
            int k = word.size();
            TrieNode* node = root;
            int idx = 0;
            while(idx < k){
                char chr = word[idx];
                if(node->child.find(chr) != node->child.end()){
                    node = node->child[chr];
                    int j = node->end;
                    // Case2: "abcxxxx" && "cba"
                    if(j != -1 && i != j && isPalindromeWord(idx + 1, k - 1, word)){
                        ans.push_back({i, j});
                    }
                }
                else{break;}
                idx++;
            }
            if(idx == k){
                vector<int> tmp;
                string tword = "";
                // Case1: "abc" && "cba", Case3: "abc" && "cbaxxxx"
                findPalindromeTrie(node, tmp, tword);
                for(auto& x: tmp){
                    if(x != i){ans.push_back({i, x});}
                }
                // Special Case: "aaaa" && ""
                if(root->end != -1 && root->end != i && isPalindromeWord(0, k - 1, word)){
                    ans.push_back({i, root->end});
                }
            }
        }

        return ans;
    }

    bool isPalindromeWord(int left, int right, string& word){
        while(left <= right){
            if(word[left] != word[right]){return false;}
            left++;
            right--;
        }
        return true;
    }

    void findPalindromeTrie(TrieNode* node, vector<int>& ans, string& word){
        if(word != "" && node->end != -1 && isPalindromeWord(0, word.size() - 1, word)){
            ans.push_back(node->end);
        }
        for(auto& x: node->child){
            word = word + x.first;
            findPalindromeTrie(x.second, ans, word);
            word = word.substr(0, word.size() - 1);
        }

    }
};
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