Number of closed islands

class Solution {
    public int closedIsland(int[][] grid) {
        int m=grid.length;
        int n=grid[0].length;
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(i==0||i==m-1||j==0||j==n-1){
                    if(grid[i][j]==0){
                        dfs(i,j,grid,m,n);
                    }
                }
            }
        }
        
        int answer=0;
        for(int i=1;i<m-1;i++){
            for(int j=1;j<n-1;j++){
                if(grid[i][j]==0){
                    dfs(i,j,grid,m,n);
                    answer++;
                }
            }
        }
       return answer; 
    }
    private void dfs(int cr,int cc,int[][] grid,int m,int n){
        if(cr<0||cr>=m||cc<0||cc>=n||grid[cr][cc]!=0){
            return;
        }
        grid[cr][cc]=1;
        dfs(cr-1,cc,grid,m,n);
        dfs(cr+1,cc,grid,m,n);
        dfs(cr,cc-1,grid,m,n);
        dfs(cr,cc+1,grid,m,n);
        return;
    }
}