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);
}
}
}