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.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isSymmetric(TreeNode root) {
        if(root==null)
        	return true;
        return judge(root.left, root.right);
    }
    public boolean judge(TreeNode left,TreeNode right){
        if(left==null&&right==null)
			return true;
		if(left==null||right==null)
			return false;
		if(left.val!=right.val)
			return false;
		return judge(left.left, right.right)&&judge(left.right, right.left);
	}
}