package binaryTree.visitor; import binaryTree.*; import ordering.*; /** * Deletes the given input IOrdered from the host tree. *
Invariant: host contains unique IOrdered objects and satisfies the binary search tree property. Post: host does not contains the input IOrdered. Algorithm: Case host is empty: returns null because there is nothing to remove. Case host is not empty: if input < root ask the host's left subtree to delete the input; else if root < input ask the host's right subtree to delete the input else (at this point, input == root) if host is a leaf ask the host to remove its root; else if host's left subtree is empty (i.e. right subtree is NOT empty) { ask host to replace the root with the minimum value of its right subtree; ask host's right subtree to delete this minimum value; } else (at this point, the left subtree must NOT be empty) { ask host to replace the root with the maximum value of its left subtree; ask host's right subtree to delete this maximum value; }
NOTE: The above algorithm is not "object-oriented" enough. Can you think of a more OO way? Hint: Think of the OO algorithm to check for a leaf. * @author Dung X. Nguyen * @since 11/12/99 Copyright 1999 - All rights reserved. */ public class BSTDeleter implements IVisitor { public final static BSTDeleter Singleton = new BSTDeleter (); private BSTDeleter() { } /** * Returns null. * @param host satifies BST property * @param input an IOrdered. */ public Object emptyCase(BiTree host, Object input) { return null; // there is nothing to delete. } /** * Returns input if host contains input, otherwise returns null. * @param host satifies BST property * @param input an IOrdered. */ public Object nonEmptyCase(BiTree host, Object input) { IOrdered root = (IOrdered)host.getRootDat(); IOrdered inp = (IOrdered)input; int inpCompRoot = inp.compare (root); if (inpCompRoot == IOrdered.LESS) { return host.getLeftSubTree ().execute(this, input); } else if (inpCompRoot == IOrdered.GREATER) { return host.getRightSubTree ().execute(this, input); } // At this point, root == inp. Why? boolean isLeftEmpty = ((Boolean)host.getLeftSubTree ().execute(EmptyChecker.Singleton, this)).booleanValue (); boolean isRightEmpty = ((Boolean)host.getRightSubTree ().execute(EmptyChecker.Singleton, this)).booleanValue (); if (isLeftEmpty && isRightEmpty) { return host.remRoot (); } // At this point, at least one of the subtrees is not empty. Why? BiTree minTree = (BiTree)(isLeftEmpty? host.getRightSubTree ().execute (MinTreeFinder.Singleton, null) : host.getLeftSubTree ().execute (MaxTreeFinder.Singleton, null)); Object min = minTree.getRootDat(); host.setRootDat (min); minTree.execute (this, min); return input; } }