插入和搜索二进制搜索树

Question

I am having trouble with my BST. The BST which I am supposed to build has an implicit 'rank' in which the Nodes are sorted by. For instance, when inserting a new Node I am given a value to store in the Node and the rank in which to insert it. In other words the BST is supposed to store a sequence. My insert function appears to work but small bugs appear occasionally and the select function throws a NullPointerException when it should not.

Node.java

RBST.java

RBSTTest.java

public void insertNormal(int team, int rank)
{
    root = insertNormal(root, team, rank);
}

 /**
 * Insert the data team at position rank into node T. This is the normal
 * insert routine without any balancing.
 */
private Node insertNormal(Node T, int team, int rank)
{
    assert (rank >= 1 && rank <= T.getSize() + 1) : "rank should be between 1 and size of the tree <"
        + (T.getSize() + 1) + ">";

    if (T == null)
    {
        return new Node(team);
    }

    if (getRank(T) >= rank)
    {
        T.setLeft(insertNormal(T.getLeft(), team, rank));
    }
    else
    {
        T.setRight(insertNormal(T.getRight(), team, rank));
    }
    T.incSize();

    return T;
}


public Node select(int rank)
{
    return select(root, rank);
}

/**
 * The select method that returns the node in the tree at position rank.
 */
private Node select(Node T, int rank)
{
    if (T == null || getRank(T) == rank)
        return T;

    assert (rank >= 1 && rank <= T.getSize()) : "rank should be between 1 and size of the tree <" + T.getSize()
        + "> ";

    if (getRank(T) > rank)
        T = T.getLeft();
    else
        T = T.getRight();

    return select(T, rank);
}

public int getRank(Node T)
{
    return (T.getLeft() == null ? 1 : T.getLeft().getSize() + 1);
}

Answer 1

The binary search tree is constructed a bit strangely. Apparently you know the rank of the teams beforehand. In that case, you would normally store the rank with the team in the Node . Then you can easily retrieve the rank later. That way, you are also not forced to add the teams in order of their ranks.

Your current getRank() method currently does not actually retrieve the rank. Note that if you add teams in order of their rank now, you will get a very lob-side tree with the sub-tree always on the right side, with the left side always null . That is also likely where your faults come from: getRank() will always return 1.