struct TrieNode {

TrieNode* children[26];

int wordEndCnt;

string word;

TrieNode() : wordEndCnt(0), word("") {

for (int i = 0; i < 26; ++i)

children[i] = nullptr;

}

};

class Solution {

void addWord(TrieNode* node, const string& word) {

// simple trie insertion

TrieNode* cur = node;

int idx;

for (int i = 0; i < word.size(); ++i) {

idx = word[i] - 'a';

if (cur->children[idx] == nullptr)

cur->children[idx] = new TrieNode;

cur = cur->children[idx];

}

// increase the word end counter

++cur->wordEndCnt;

cur->word = word;

}

void solve(vector<vector<char>>& board, int ROWS, int COLS, int r, int c, vector<string>& result, TrieNode* cur) {

// boundary check

if (r < 0 || r >= ROWS || c < 0 || c >= COLS)

return;

// visit check

if (board[r][c] == '$')

return;

// existence check

int idx = board[r][c] - 'a';

if (cur->children[idx] == nullptr)

return;

cur = cur->children[idx];

if (cur->wordEndCnt > 0) {

// found word

result.push_back(cur->word);

--cur->wordEndCnt;

}

// mark as visited and backtrack

char origin = board[r][c];

board[r][c] = '$';

solve(board, ROWS, COLS, r + 1, c, result, cur);

solve(board, ROWS, COLS, r - 1, c, result, cur);

solve(board, ROWS, COLS, r, c + 1, result, cur);

solve(board, ROWS, COLS, r, c - 1, result, cur);

board[r][c] = origin;

}

vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {

TrieNode* root = new TrieNode;

for (auto w : words) {

addWord(root, w);

}

int ROWS = board.size(), COLS = board[0].size();

vector<string> result;

// check every cells

for (int i = 0; i < ROWS; ++i)

for (int j = 0; j < COLS; ++j)

solve(board, ROWS, COLS, i, j, result, root);

return result;

}

};