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

IsSymmetricTree.java

public boolean isSymmetric(TreeNode root) {
    return isSymmetricHelper(root, root);
}

private static boolean isSymmetricHelper(TreeNode root1, TreeNode root2) {
    if(root1 == null && root2 == null)
        return true;
    if(root1 == null || root2 == null)
        return false;

    return  root1.val == root2.val &&
            isSymmetricHelper(root1.left, root2.right) &&
            isSymmetricHelper(root1.right, root2.left);
}