Leetcode 200 Number of Islands DFS

class Solution {
public:
    int m, n;
    bool is_in(int x, int y)
    {
        return (x < m ) && (x >= 0) && (y < n ) && (y >= 0);
    }

    void dfs(std::vector<std::vector<char>> &board, int x, int y)
    {
        board[x][y] = ‘0‘;
        int dir[4][2] = { { 1, 0 }, { -1, 0 }, { 0, 1 }, { 0, -1 } };
        for (int i = 0; i < 4; ++i){
            int tx = x + dir[i][0];
            int ty = y + dir[i][1];
            if (is_in(tx, ty) && board[tx][ty] == ‘1‘)
            {
                dfs(board, tx, ty);
            }
        }
    }

    int numIslands(std::vector<std::vector<char>>& grid)
    {
        m = grid.size();
        if (m == 0) return 0;
        n = grid[0].size();
        if (n == 0) return 0;
        int ans = 0;
        for (int i = 0; i < m; ++i){
            for (int j = 0; j < n; ++j){
                if (grid[i][j] == ‘1‘){
                    dfs(grid, i, j);
                    ans++;
                }
            }
        }
        return ans;
    }
};