二叉樹的層次遍歷

題目原型：

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]
]

略

	public ArrayList> levelOrder(TreeNode root)
	{
		ArrayList> list = new ArrayList>();
		ArrayList nodeSet = new ArrayList();
		ArrayList tmp ;
		ArrayList numSet ;
		if(root!=null)
		{
			nodeSet.add(root);
			while(nodeSet.size()>0)
			{
				tmp = new ArrayList();
				numSet = new ArrayList();
				//添加到list中
				for(TreeNode tn : nodeSet)
					numSet.add(tn.val);
				list.add(numSet);
				//求下一層的節點
				for(TreeNode it : nodeSet)
				{
					if(it.left!=null)
						tmp.add(it.left);
					if(it.right!=null)
						tmp.add(it.right);
				}
				nodeSet = tmp;
			}
		}
		return list;

	}