101. Symmetric Tree（判断二叉树是否对称）

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

Note:
Bonus points if you could solve it both recursively and iteratively.

 class Solution {
     public boolean isSymmetric(TreeNode root) {
         if(root ==null) return true;
         return  isSy(root.left,root.right);
     }
     public boolean isSy(TreeNode s,TreeNode t) {
         if(s == null || t == null) return s==t;

         if(s.val != t.val) return false;

         return isSy(s.left,t.right) && isSy(s.right,t.left);
     }
 }