<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:
    vector<TrieNode*> children;
    int id;

public:
    TrieNode(){
        children.assign(26,NULL);
        id=-1;
    } 
};

class Solution {
public:

    void insert(TrieNode* dict,string word,int i){
        TrieNode * ptr=dict;
        for(int j=word.size()-1;j>=0;j--){
            int x= word[j]-'a';
            if(!ptr->children[x]){
                ptr->children[x]=new TrieNode();
            }           
            ptr=ptr->children[x];           
        }
        ptr->id=i;
    }

    vector<int> searchWords(TrieNode* root){
        vector<int> res;
        stack<TrieNode*> dfs;
        dfs.push(root);
        while(!dfs.empty()){
            TrieNode* curr=dfs.top(); dfs.pop();
            if(curr->id!=-1) res.push_back(curr->id);
            for(int i=0;i<26;i++){
                if(curr->children[i]) dfs.push(curr->children[i]);
            }
        }

        return res;
    }

    bool isPalindrome(string s,int i,int j){
        if(s==" ") return true;
        while(i<=j){
            if(s[i]!=s[j]){
                return false;
            }
            i++;
            j--;
        }
        return true;
    }

    vector<vector<int>> palindromePairs(vector<string>& words) {
        TrieNode* dict=new TrieNode();
        for(int id=0;id<words.size();id++){
            insert(dict,words[id],id);
        }

        vector<vector<int>> pairs;

        
        for(int i=0;i<words.size();i++){
            string w=words[i];
            TrieNode* ptr=dict;
            int j=0;
            for(;j<w.size() && ptr;j++ ){
                int x=w[j]-'a';
                if(ptr->id!=-1 && ptr->id!=i && isPalindrome(w,j,w.size()-1))
                    pairs.push_back({i,ptr->id});       
                ptr=ptr->children[x];     
            }

            if (!ptr) continue; 
            for(int id:searchWords(ptr)){
                if(i!=id && isPalindrome(words[id],0,words[id].size()-j-1)) 
                    pairs.push_back({i,id});
            }
        }

        return pairs;
    }


};
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