Solution.cpp

class Solution {
public:
    bool exist(vector<vector<char>>& board, string word) {
        int m = board.size(), n = board[0].size();
        int dirs[5] = {-1, 0, 1, 0, -1};
        function<bool(int, int, int)> dfs = [&](int i, int j, int k) -> bool {
            if (k == word.size() - 1) {
                return board[i][j] == word[k];
            }
            if (board[i][j] != word[k]) {
                return false;
            }
            char c = board[i][j];
            board[i][j] = '0';
            for (int u = 0; u < 4; ++u) {
                int x = i + dirs[u], y = j + dirs[u + 1];
                if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] != '0' && dfs(x, y, k + 1)) {
                    return true;
                }
            }
            board[i][j] = c;
            return false;
        };
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (dfs(i, j, 0)) {
                    return true;
                }
            }
        }
        return false;
    }
};