Career cup 4.8 - Subtree detection

SubtreeFinder.java

package Chapter4;

public class SubtreeFinder {
    public static <T extends Comparable<T>> boolean isIdentical(TreeNode<T> r1,
	    TreeNode<T> r2) {
	if (r1 == null && r2 == null)
	    return true;
	if (r1 == null || r2 == null)
	    return false;
	if (r1.key != r2.key)
	    return false;
	return isIdentical(r1.left, r2.left) && isIdentical(r1.right, r2.right);
    }

    public static <T extends Comparable<T>> boolean isSubtree(TreeNode<T> n1,
	    TreeNode<T> n2) {
	// tree with null root is subtree of all other trees
	if (n2 == null)
	    return true;

	// root is null, but target tree still has nodes
	if (n1 == null)
	    return false;
	
	if (isIdentical(n1, n2))
	    return true;
	return isSubtree(n1.left, n2) || isSubtree(n1.right, n2);
    }

    public static void main(String[] args) {
	/*
	 * _______7______ / \ __10__ ___2 / \ / 4 3 _8 \ / 1 11
	 */
	Integer[] pre = { 7, 10, 4, 3, 1, 2, 8, 11 };
	Integer[] in = { 4, 10, 3, 1, 7, 11, 8, 2 };
	BinaryTree<Integer> bt = BinaryTreeBuilder.buildTree(pre, in);

	/*
	 * ___2 / _8 / 11
	 */
	Integer[] sub_pre = { 2, 8, 11 };
	Integer[] sub_in = { 11, 8, 2 };
	BinaryTree<Integer> st1 = BinaryTreeBuilder.buildTree(sub_pre, sub_in);
	BinaryTree<Integer> st2 = new BinaryTree<>(new TreeNode<Integer>(8));

	System.out.println("is st1 subtree of bt: "
		+ isSubtree(bt.root(), st1.root()));
	System.out.println("is st2 subtree of bt: "
		+ isSubtree(bt.root(), st2.root()));
	System.out.println("is null root subtree of bt: "
		+ isSubtree(bt.root(), null));
    }
}