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。

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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);         }     } }