<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 Solution {
public:
    struct TrieNode {
        vector<shared_ptr<TrieNode>> children;
        bool isEnd{false};

        TrieNode() {
            for (int i = 0; i < 26; i++) {
                children.push_back(nullptr);
            }
        }
    };

    void insertTrie(shared_ptr<TrieNode>& root, string& word) {
        auto curr = root;
        for (auto c : word) {
            if (curr->children[c-'a'] == nullptr) {
                curr->children[c-'a'] = make_shared<TrieNode>();                
            }
            
            curr = curr->children[c-'a'];            
        }

        curr->isEnd = true;
    }

    void dfs(vector<vector<char>>& board, int r, int c, string& preStr, shared_ptr<TrieNode>& currNode, vector<vector<int>>& visited, unordered_set<string>& res) {
        if (r < 0 || r >= size(board) || c < 0 || c >= size(board[0])) {
            return;
        }

        if (visited[r][c] == 1) {
            return;
        }
    
        if (currNode->children[board[r][c] - 'a'] == nullptr) {
            return;
        }

        //if (currNode->isEnd) {
        // NOTE: must check children
        if (currNode->children[board[r][c] - 'a']->isEnd) {    
            res.insert(preStr + board[r][c]);
            //return;
        }

        visited[r][c] = 1;
        for (auto& d : dir) {
            auto nextPreStr = preStr + board[r][c];
            dfs(board, r + d[0], c + d[1], nextPreStr, currNode->children[board[r][c] - 'a'], visited, res);
        }


        // backtrack, reset visited
        visited[r][c] = 0;
    }
    
    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        // Build trie
        auto trieRoot = make_shared<TrieNode>();
        for (auto& w : words) {
            insertTrie(trieRoot, w);
        }

        // DFS to find all the words from board
        unordered_set<string> res;
        //string preStr;
        vector<vector<int>> visited(size(board), vector<int>(size(board[0]), 0));

        for (int r = 0; r < size(board); r++) {
            for (int c = 0; c < size(board[0]); c++) {
                string preStr;
                dfs(board, r, c, preStr, trieRoot, visited, res);
            }
        }
        //dfs(board, 0, 0, preStr, trieRoot, visited, res);

        vector<string> allWords(begin(res), end(res));
        return allWords;
    }

private:
    vector<vector<int>> dir{ {-1, 0}, {1, 0}, {0, -1}, {0, 1} };
};
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