<p>Given an <code>m x n</code> <code>board</code>&nbsp;of characters and a list of strings <code>words</code>, return <em>all words on the board</em>.</p>

<p>Each word must be constructed from letters of sequentially adjacent cells, where <strong>adjacent cells</strong> are horizontally or vertically neighboring. The same letter cell may not be used more than once in a word.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/07/search1.jpg" style="width: 322px; height: 322px;" />
<pre>
<strong>Input:</strong> board = [[&quot;o&quot;,&quot;a&quot;,&quot;a&quot;,&quot;n&quot;],[&quot;e&quot;,&quot;t&quot;,&quot;a&quot;,&quot;e&quot;],[&quot;i&quot;,&quot;h&quot;,&quot;k&quot;,&quot;r&quot;],[&quot;i&quot;,&quot;f&quot;,&quot;l&quot;,&quot;v&quot;]], words = [&quot;oath&quot;,&quot;pea&quot;,&quot;eat&quot;,&quot;rain&quot;]
<strong>Output:</strong> [&quot;eat&quot;,&quot;oath&quot;]
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2020/11/07/search2.jpg" style="width: 162px; height: 162px;" />
<pre>
<strong>Input:</strong> board = [[&quot;a&quot;,&quot;b&quot;],[&quot;c&quot;,&quot;d&quot;]], words = [&quot;abcb&quot;]
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>m == board.length</code></li>
	<li><code>n == board[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 12</code></li>
	<li><code>board[i][j]</code> is a lowercase English letter.</li>
	<li><code>1 &lt;= words.length &lt;= 3 * 10<sup>4</sup></code></li>
	<li><code>1 &lt;= words[i].length &lt;= 10</code></li>
	<li><code>words[i]</code> consists of lowercase English letters.</li>
	<li>All the strings of <code>words</code> are unique.</li>
</ul>

class Trie {
public:
    vector<Trie*> child;
    bool isend;
Trie(){
        this->child.resize(26,nullptr);
        this->isend=false;
    }
    void insert(string &s){
        auto head = this;
        for(auto &x:s){
            if(head->child[x-'a']==nullptr){
                Trie *child = new Trie();
                head->child[x-'a'] = child;
            }
            head = head->child[x-'a'];
        }
        head->isend = true;
    }
    bool search(string &s){
        auto head = this;
        for(auto &x:s){
            if(head->child[x-'a']==nullptr)return false;
            head = head->child[x-'a'];
        }
        if(head->isend)return true;
        return false;
    }
};
class Solution {
public:
    set<string> ans;
    Trie* root = new Trie();
    int N, M;
    vector<vector<bool>> vis;
    // unordered_map<string, bool> done;
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        for(auto &w:words){
            root->insert(w);
        }
        N = board.size();
        M = board[0].size();
        string curr;
        vis.resize(N, vector<bool> (M, false));
        for(int i = 0; i<N; i++){
            for(int j= 0; j<M; j++){
                help(board, i, j, root, curr);
            }
        }
        vector<string> ANS;
        for(auto &x:ans){
            ANS.push_back(x);
        }
        return ANS;
    }
    void help(vector<vector<char>>& board, int i, int j, Trie *root, string& ans_){
        if(i<0 || i>N-1 || j<0 || j>M-1 || vis[i][j] || !root->child[board[i][j]-'a'])return;
        ans_+=board[i][j];
        root=root->child[board[i][j]-'a'];
        if(root->isend){
            ans.insert(ans_);
        }
        vis[i][j]=true;
        help(board, i+1, j, root, ans_);
        help(board, i-1, j, root, ans_);
        help(board, i, j+1, root, ans_);
        help(board, i, j-1, root, ans_);
        vis[i][j]=false;
        ans_ = ans_.substr(0, ans_.size()-1);
        return;
    }
};
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