Binary Tree Level Order Traversal
	
	/**
	 * Given a binary tree, return the level order traversal of its nodes'
	 * values. (ie, from left to right, level by level).
	 * 
	 * For example: Given binary tree {3,9,20,#,#,15,7}
	 * 
				3
			   / \
			  9  20
			    /  \
			   15   7
	 * 
	 * return its level order traversal as:
	 * 
		    [
			  [3],
			  [9,20],
			  [15,7]
			]
	 * 
	 * 
	 * */