Binary Tree Post-order Traversal without recursion (using stack)
	
	/**
	 * 			  5
	 * 		  /      \
	 *       3        6
	 *      / \      / \
	 *     2   4    7   8
	 *        / \  / \
	 *       9  0 1   11
	 * 
	 * Output:  2 9 0 4 3    1 11 7 8 6    5 
	 * 
	 * */