boolean checkEqual(TreeNode a, TreeNode b) {
        if (a == null && b == null) return true;
        return a != null && b != null && a == b && checkEqual(a.left, b.left) && checkEqual(a.right, b.right);
    }
Given a binary tree, check whether it is a mirror of itself (ie, symmetric around its center).
src/problems/easy/SymmetricTree.java