public static List<Character> getAdjacent(List<String> grid, int i,int j){
    return Arrays.asList(grid.get(i+1).charAt(j), grid.get(i-1).charAt(j),
            grid.get(i).charAt(j+1), grid.get(i).charAt(j-1));
    }
    
    public static int getInt(char c){
        return c - '0';
        }
       
    public static List<String> cavityMap(List<String> grid) {
    int n = grid.size();
    List<String> new_grid = new ArrayList<>();
    
    for(int i = 0; i < n; i++){
        String line = "";
        for(int j = 0; j < n; j++){
            char c = grid.get(i).charAt(j);
            if(i == 0 || i == n-1 || j == 0 || j == n-1){
                line += c;
                continue;
                }
            boolean is_cavity = true;
            
            
            for(char adj: getAdjacent(grid, i, j)){
                if(getInt(adj) >= getInt(c))
                  is_cavity = false; 
                }
            if(is_cavity)
                c = 'X';
            
            line += c;
            
            }
        new_grid.add(line);
        }
    
    return new_grid;

    }