Symmetric Tree，对称树

问题描述：

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

For example, this binary tree [1,2,2,3,4,4,3] is symmetric:

    1
   / 
  2   2
 /  / 
3  4 4  3

But the following [1,2,2,null,3,null,3] is not:

    1
   / 
  2   2
      
   3    3

算法分析：和same tree比较像，都可以用递归方法来解决。

public class SymmeticTree 
{
	public boolean isSymmetric(TreeNode root) 
	{
		if (root == null) 
		{
			return true;
		}
		return isSymmetric(root.left, root.right);
	}

	public boolean isSymmetric(TreeNode left, TreeNode right) 
	{
		if (left == null && right == null) 
		{
			return true;
		}
		else if ((left == null && right != null)
				|| (right == null && left != null) || (left.val != right.val))
		{
			return false;
		}
		else
		{
			return isSymmetric(left.left, right.right)
					&& isSymmetric(left.right, right.left);
		}
	}
}