avltree类扩展了bst类以重写insert和delete方法以在必要时重新平衡树。下面的代码给出了 avltree 类的完整源代码。
package demo;
public class AVLTree<E extends Comparable<E>> extends BST<E> {
/** Create an empty AVL tree */
public AVLTree() {}
/** Create an AVL tree from an array of objects */
public AVLTree(E[] objects) {
super(objects);
}
@Override /** Override createNewNode to create an AVLTreeNode */
protected AVLTreeNode<E> createNewNode(E e) {
return new AVLTreeNode<E>(e);
}
@Override /** Insert an element and rebalance if necessary */
public boolean insert(E e) {
boolean successful = super.insert(e);
if (!successful)
return false; // e is already in the tree
else {
balancePath(e); // Balance from e to the root if necessary
}
return true; // e is inserted
}
/** Update the height of a specified node */
private void updateHeight(AVLTreeNode<E> node) {
if (node.left == null && node.right == null) // node is a leaf
node.height = 0;
else if (node.left == null) // node has no left subtree
node.height = 1 + ((AVLTreeNode<E>)(node.right)).height;
else if (node.right == null) // node has no right subtree
node.height = 1 + ((AVLTreeNode<E>)(node.left)).height;
else
node.height = 1 + Math.max(((AVLTreeNode<E>)(node.right)).height, ((AVLTreeNode<E>)(node.left)).height);
}
/** Balance the nodes in the path from the specified
* node to the root if necessary
*/
private void balancePath(E e) {
java.util.ArrayList<TreeNode<E>> path = path(e);
for (int i = path.size() - 1; i >= 0; i--) {
AVLTreeNode<E> A = (AVLTreeNode<E>)(path.get(i));
updateHeight(A);
AVLTreeNode<E> parentOfA = (A == root) ? null : (AVLTreeNode<E>)(path.get(i - 1));
switch (balanceFactor(A)) {
case -2:
if (balanceFactor((AVLTreeNode<E>)A.left) <= 0) {
balanceLL(A, parentOfA); // Perform LL rotation
}
else {
balanceLR(A, parentOfA); // Perform LR rotation
}
break;
case +2:
if (balanceFactor((AVLTreeNode<E>)A.right) >= 0) {
balanceRR(A, parentOfA); // Perform RR rotation
}
else {
balanceRL(A, parentOfA); // Perform RL rotation
}
}
}
}
/** Return the balance factor of the node */
private int balanceFactor(AVLTreeNode<E> node) {
if (node.right == null) // node has no right subtree
return -node.height;
else if (node.left == null) // node has no left subtree
return +node.height;
else
return ((AVLTreeNode<E>)node.right).height - ((AVLTreeNode<E>)node.left).height;
}
/** Balance LL (see Figure 26.2) */
private void balanceLL(TreeNode<E> A, TreeNode<E> parentOfA) {
TreeNode<E> B = A.left; // A is left-heavy and B is left-heavy
if (A == root) {
root = B;
}
else {
if (parentOfA.left == A) {
parentOfA.left = B;
}
else {
parentOfA.right = B;
}
}
A.left = B.right; // Make T2 the left subtree of A
B.right = A; // Make A the left child of B
updateHeight((AVLTreeNode<E>)A);
updateHeight((AVLTreeNode<E>)B);
}
/** Balance LR (see Figure 26.4) */
private void balanceLR(TreeNode<E> A, TreeNode<E> parentOfA) {
TreeNode<E> B = A.left; // A is left-heavy
TreeNode<E> C = B.right; // B is right-heavy
if (A == root) {
root = C;
}
else {
if (parentOfA.left == A) {
parentOfA.left = C;
}
else {
parentOfA.right = C;
}
}
A.left = C.right; // Make T3 the left subtree of A
B.right = C.left; // Make T2 the right subtree of B
C.left = B;
C.right = A;
// Adjust heights
updateHeight((AVLTreeNode<E>)A);
updateHeight((AVLTreeNode<E>)B);
updateHeight((AVLTreeNode<E>)C);
}
/** Balance RR (see Figure 26.3) */
private void balanceRR(TreeNode<E> A, TreeNode<E> parentOfA) {
TreeNode<E> B = A.right; // A is right-heavy and B is right-heavy
if (A == root) {
root = B;
}
else {
if (parentOfA.left == A) {
parentOfA.left = B;
}
else {
parentOfA.right = B;
}
}
A.right = B.left; // Make T2 the right subtree of A
B.left = A;
updateHeight((AVLTreeNode<E>)A);
updateHeight((AVLTreeNode<E>)B);
}
/** Balance RL (see Figure 26.5) */
private void balanceRL(TreeNode<E> A, TreeNode<E> parentOfA) {
TreeNode<E> B = A.right; // A is right-heavy
TreeNode<E> C = B.left; // B is left-heavy
if (A == root) {
root = C;
}
else {
if (parentOfA.left == A) {
parentOfA.left = C;
}
else {
parentOfA.right = C;
}
}
A.right = C.left; // Make T2 the right subtree of A
B.left = C.right; // Make T3 the left subtree of B
C.left = A;
C.right = B;
// Adjust heights
updateHeight((AVLTreeNode<E>)A);
updateHeight((AVLTreeNode<E>)B);
updateHeight((AVLTreeNode<E>)C);
}
@Override /** Delete an element from the AVL tree.
* Return true if the element is deleted successfully
* Return false if the element is not in the tree */
public boolean delete(E element) {
if (root == null)
return false; // Element is not in the tree
// Locate the node to be deleted and also locate its parent node
TreeNode<E> parent = null;
TreeNode<E> current = root;
while (current != null) {
if (element.compareTo(current.element) < 0) {
parent = current;
current = current.left;
}
else if (element.compareTo(current.element) > 0) {
parent = current;
current = current.right;
}
else
break; // Element is in the tree pointed by current
}
if (current == null)
return false; // Element is not in the tree
// Case 1: current has no left children (See Figure 25.10)
if (current.left == null) {
// Connect the parent with the right child of the current node
if (parent == null) {
root = current.right;
}
else {
if (element.compareTo(parent.element) < 0)
parent.left = current.right;
else
parent.right = current.right;
// Balance the tree if necessary
balancePath(parent.element);
}
}
else {
// Case 2: The current node has a left child
// Locate the rightmost node in the left subtree of
// the current node and also its parent
TreeNode<E> parentOfRightMost = current;
TreeNode<E> rightMost = current.left;
while (rightMost.right != null) {
parentOfRightMost = rightMost;
rightMost = rightMost.right; // Keep going to the right
}
// Replace the element in current by the element in rightMost
current.element = rightMost.element;
// Eliminate rightmost node
if (parentOfRightMost.right == rightMost)
parentOfRightMost.right = rightMost.left;
else
// Special case: parentOfRightMost is current
parentOfRightMost.left = rightMost.left;
// Balance the tree if necessary
balancePath(parentOfRightMost.element);
}
size--;
return true; // Element inserted
}
/** AVLTreeNode is TreeNode plus height */
protected static class AVLTreeNode<E extends Comparable<E>> extends BST.TreeNode<E> {
protected int height = 0; // New data field
public AVLTreeNode(E e) {
super(e);
}
}
}
avltree 类扩展了bst。与 bst 类一样，avltree 类有一个无参构造函数，用于构造一个空的 avltree（第 5 行），以及一个从元素数组创建初始 avltree 的构造函数（第 8-10 行） .
bst类中定义的createnewnode()方法创建一个treenode。重写此方法以返回 avltreenode（第 13-15 行）。
avltree中的insert方法在第18-27行被覆盖。该方法首先调用bst中的insert方法，然后调用balancepath(e)（第23行）来确保树是平衡的。
balancepath方法首先获取从包含元素e的节点到根节点的路径上的节点（第45行）。对于路径中的每个节点，更新其高度（第 48 行），检查其平衡系数（第 51 行），并在必要时执行适当的旋转（第 51-67 行）。
第 82-178 行定义了四种执行旋转的方法。每个方法都使用两个treenode 参数（a 和 parentofa）进行调用，以在节点 a 处执行适当的旋转。帖子中的附图说明了如何执行每次旋转。旋转后，节点a、b和c的高度更新（第98、125、148、175行）。
avltree中的delete方法在第183-248行被重写。该方法与bst类中实现的方法相同，只是在两种情况下需要在删除后重新平衡节点（第218、243行）。
