难得力扣数据放水了hh

执行用时：4 ms, 在所有 C++ 提交中击败了88.84%的用户
内存消耗：8.2 MB, 在所有 C++ 提交中击败了90.43%的用户

class Solution {
public:
    int n, m;
    vector<string> res;
    vector<vector<char>> g;

    bool dfs(string &s, int x, int y, int u) {
        if (u == s.size()) return true;

        int dx[] = {-1, 0, 1, 0}, dy[] = {0, 1, 0, -1};
        g[x][y] = '.';
        for (int i = 0; i < 4; i++) {
            int a = x + dx[i], b = y + dy[i];
            if (a >= 0 && a < n && b >= 0 && b < m && g[a][b] == s[u]) {
                g[a][b] = '.';
                if (dfs(s, a, b, u + 1)) {
                    g[a][b] = s[u];
                    g[x][y] = s[u - 1];
                    return true;
                }
                g[a][b] = s[u];
            }
        }
        g[x][y] = s[u - 1];
        return false;
    }

    vector<string> findWords(vector<vector<char>>& board, vector<string>& words) {
        n = board.size(), m = board[0].size();
        g = board;

        for (auto s : words) {
            bool flag = true;
            for (int i = 0; i < n && flag; i++) {
                for (int j = 0; j < m; j++)
                    if (g[i][j] == s[0])
                        if (dfs(s, i, j, 1)) {
                            res.push_back(s);
                            flag = false;
                            break;
                        }
            }
        }
        return res;
    }
};