<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 Node {
    public:
        char val;
        bool wordEnd = false;
        bool hasNext = false;
        Node* children[26] = {};
        Node() {
            val = '\0';
        }
        Node(char _val) {
            val = _val;
        }
    };


class Trie {
public:
    Node* root;

    Trie() {
        this->root = new Node();
    }
    
    void insert(string word) {
        auto curNode = root;
        for (auto c: word) {
            int index = c - 97;
            if (curNode->children[index] == NULL) {
                curNode->children[index] = new Node(c);
                curNode->hasNext = true;
            }
            curNode = curNode->children[index];
        }
        curNode->wordEnd = true;
    }
    
    int search(string word) {
        auto curNode = root;
        for (auto c: word) {
            int index = c - 97;
            if (curNode->children[index] == nullptr) {
                return 0;
            }
            curNode = curNode->children[index];
        }
        if (curNode->wordEnd) return 2;
        else return 1;
        return curNode->wordEnd;
    }
};

class Solution {
public:
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        Trie wordTrie;
        for (auto word: words) {
            wordTrie.insert(word);
        }
        unordered_set<string> foundWords;
        unordered_set<int> visited;
        for (int i = 0; i < board.size(); i++) {
            for (int j = 0; j < board[0].size(); j++) {
                dfs(wordTrie.root, board, foundWords, visited, "", i, j);
                visited = {};
            }
        }
        vector<string> res;
        for (auto c: foundWords) {
            res.push_back(c);
        }
        return res;
    }

    void dfs(Node* trieNode, vector<vector<char>>& board, unordered_set<string>& foundWords, unordered_set<int>& visited, string curStr, int r, int c) {

        char curChar = board[r][c];
        curStr += curChar;
        if (trieNode->children[curChar - 97] != nullptr) {
            auto nextNode = trieNode->children[curChar - 97];
            if (nextNode->wordEnd) {
                foundWords.insert(curStr);
            }
            if (nextNode->hasNext) {
                visited.insert(r * board[0].size() + c);
                //cout << curStr << " is prefix" << endl;
                if (r < board.size() - 1 && !visited.contains((r+1)*board[0].size() + c)) dfs(nextNode, board, foundWords, visited, curStr, r+1, c);
                if (r > 0 && !visited.contains((r-1)*board[0].size() + c)) dfs(nextNode, board, foundWords, visited, curStr, r-1, c);
                if (c < board[0].size() - 1 && !visited.contains((r*board[0].size() + c+1))) dfs(nextNode, board, foundWords, visited, curStr, r, c+1);
                if (c > 0 && !visited.contains(r*board[0].size() + c-1)) dfs(nextNode, board, foundWords, visited, curStr, r, c-1);
            }
        }
        visited.erase(r * board[0].size() + c);
        //cout << endl;
    }
};
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