Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).

For example, this binary tree [5,3,3,1,4,4,1] is symmetric:

     5
   /  \
  3     3
 / \   / \
1   4 4   1

or
     5
   /  \
  3     3
 /       \
1         1

  public class SymmetricTree {
    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;
        if (left == null || right == null)
            return false;
        if (left.val != right.val)
            return false;
        return isSymmetric(left.left, right.right) && 
        	   isSymmetric(left.right, right.left);
    }
}