Symmetric Tree（中心對稱樹）

題目原型：

Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree is symmetric:

    1
   / \
  2   2
 / \ / \
3  4 4  3

But the following is not:

    1
   / \
  2   2
   \   \
   3    3

根據例子我們發現，每一層，給定兩個指針分別從左端和右端開始，只有當左端的左子樹的值等於右端的右子樹的值，並且左端的右子樹的值等於右端左子樹的值時，此樹便稱為中心對稱樹（Symmetric Tree）。

	public boolean isSymmetric(TreeNode root)
	{
		if(root==null)
			return true;
		ArrayList list = new ArrayList();
		list.add(root);
		boolean isFirst = true;
		while(list.size()>0)
		{
			if(list.size()%2==1&&isFirst==false)
				return false;
			for(int i = 0,j=list.size()-1;i<=j;i++,j--)
			{
				TreeNode one = list.get(i);
				TreeNode two = list.get(j);
				if(one.left!=null&&two.right!=null)
				{
					if(one.left.val!=two.right.val)
						return false;
				}
				if(one.right!=null&&two.left!=null)
				{
					if(one.right.val!=two.left.val)
						return false;
				}
				if((one.left==null&&two.right!=null)||(one.left!=null&&two.right==null))
					return false;
			}
			
			//加入下一層節點
			int size = list.size();
			while(size>0)
			{
				if(list.get(0).left!=null)
					list.add(list.get(0).left);
				if(list.get(0).right!=null)
					list.add(list.get(0).right);
				list.remove(0);
				size--;
			}
			
			isFirst = false;//表示不是第一層了
		}
		return true;
	}