printPostOrder

public String printPostOrder() {
		if (root == null)
			return "";
		int maxDepth = maxDepth();
		ArrayList<ArrayList<Integer>> arrays = new ArrayList<ArrayList<Integer>>(maxDepth);
		printPostOrder(root, arrays, 0);
		String str = "";
		for (int i=0; i<arrays.size(); i++) {
			for (int j=0; j<arrays.get(i).size(); j++) {
				if (j==arrays.get(i).size()-1)
					str += arrays.get(i).get(j);
				else
					str += arrays.get(i).get(j) + ", ";
			}
		}
		return str;
	}

	private void printPostOrder(Node node, ArrayList<ArrayList<Integer>> arrays, int level) {
		if (node == null || level == maxDepth() -1)
			return;
		arrays.get(level).add(node.value);
		printPostOrder(node.left, arrays, level++);
		printPostOrder(node.right, arrays, level);
	}