Symmetric Tree

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

Follow up: Solve it both recursively and iteratively.

class Solution {
    public boolean isEqualAndOpposite(TreeNode left, TreeNode right) {
        if (left == null && right == null)
            return true;
        else if (left == null || right == null)
            return false;
        if (left.val == right.val) {
            if (isEqualAndOpposite(left.left, right.right) && isEqualAndOpposite(left.right, right.left))
                return true;
        }
        return false;
    }

    public boolean isSymmetric(TreeNode root) {
        if(root==null)
            return true;
        return isEqualAndOpposite(root.left, root.right);
    }
}