private String nodeString(Integer depth) {
		String tree = new String();
		for (int i=0; i<depth; i++) { 
			tree += "--";
		}
		tree += "(" + this.symbol + "," + this.frequency + ")\n";
		if (left!=null) {
			tree += left.nodeString(depth+1);
		}
		if (right!=null) {
			tree += right.nodeString(depth+1);
		}
		return tree;
	}