public static List<String> cavityMap(List<String> grid) {
        for (int i = 1; i < grid.size() - 1; i++) {
            for (int j = 1; j < grid.get(i).length() - 1; j++) {
                int currentDigit = Character.getNumericValue(grid.get(i).charAt(j));
                int upDigit = Character.getNumericValue(grid.get(i-1).charAt(j));
                int downDigit = Character.getNumericValue(grid.get(i+1).charAt(j));
                int leftDigit = Character.getNumericValue(grid.get(i).charAt(j-1));
                int rightDigit = Character.getNumericValue(grid.get(i).charAt(j+1));
                if (currentDigit > upDigit 
                    && currentDigit > downDigit 
                    && currentDigit > leftDigit
                    && currentDigit > rightDigit) {
                        StringBuilder newRow = new StringBuilder(grid.get(i));
                        newRow.setCharAt(j, 'X');
                        grid.set(i, newRow.toString());
                }
            }
        }
        return grid;
    }