code: public String toTreeString(int indent) {
      StringBuilder sb = new StringBuilder();
      sb.append(SPACES, 0, indent).append("[").append(master).append("]\n");
      return sb.toString();
    }
docs: Builds a string that visually shows the subtree rooted at this node (for debugging). Each line
  of the string will be indented by the given indentation amount. You should pass an indentation
  of 0 unless this method is being called as part of building a larger tree string.
  
  @param indent The indentation for each line of the tree string (usually pass 0).
  @return A string that visually shows the subtree rooted at this node.