Untitled

class Solution {
public:
    bool exist(vector<vector<char>>& board, string word) {
        int rows = board.size();
        int cols = board[0].size();
        unordered_set<string> visited;

        auto dfs = [&](int r, int c, int k, auto& dfs) -> bool {
            if (k == word.length()) {
                return true;
            }

            if (r < 0 || r >= rows || c < 0 || c >= cols || visited.count(to_string(r) + "," + to_string(c)) || board[r][c] != word[k]) {
                return false;
            }

            visited.insert(to_string(r) + "," + to_string(c));
            bool res = dfs(r + 1, c, k + 1, dfs) || dfs(r - 1, c, k + 1, dfs) || dfs(r, c + 1, k + 1, dfs) || dfs(r, c - 1, k + 1, dfs);
            visited.erase(to_string(r) + "," + to_string(c));
            return res;
        };

        unordered_map<char, int> count;
        for (char c : word) {
            count[c]++;
        }

        if (count[word[0]] > count[word.back()]) {
            reverse(word.begin(), word.end());
        }

        for (int r = 0; r < rows; r++) {
            for (int c = 0; c < cols; c++) {
                if (dfs(r, c, 0, dfs)) {
                    return true;
                }
            }
        }

        return false;        
    }
};