p101

 

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

	public boolean check(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 check(left.left, right.right) && check(left.right, right.left);
	}
}